Very simple question regarding work and energy transfer

[selftaught]

Peter M McGinnis describes work as being 'the means by which energy is transferred form one object or system to another' in Biomechanics of Sport and Exercise.

Many people explain that deformation of a car striking a solid object or a body striking the ground as the object or ground doing work on the care or body.

I can understand work in this situation in terms of W=Fd and Newton's law of action-reaction. I cannot understand work in terms of energy transfer in this situation as the object or ground possessed no energy to begin with.

Can anyone help me understand this simple question which my self teaching seems to be missing. Thanks in advance.

 

[Mapes]

Many people explain that deformation of a car striking a solid object or a body striking the ground as the object or ground doing work on the care or body.

Perhaps they were using a frame of motion in which the car or body is motionless and the object or ground is moving. This approach satisfies the definition.

 

[nasu]

The work done on a specific system can be positive (it will increase the energy of the system) or negative (it will decrease the energy of the system).
When the forces are acting against the motion (slowing down the motion) the work is considered negative (as in the case of friction forces, usually).

 

[Drakkith]

Perhaps it's that the car and the ground are BOTH doing work to deform the body of the car?

 

[selftaught]

Maybe my attempt to describe my question technically confused the issue. When a person is injured when landing from a fall, the kinetic energy accumulated during the fall is the cause of that injury. It is absorbed by the body and exceeded the bodies physiological tolerance levels. Some people describe the injury process in terms of work being done on the person's body. Given the Earth has no mechanical energy, how can it transfer energy to the person's body. For the respondent that suggested they might be using a frame of reference whereby the person's body is stationary and the ground is moving? When have you ever known for the Earth to move towards a stationary person? To the respondent suggesting can be +ve or -ve - this is still not addressing the energy transfer issue. To the respondent suggesting work is being done on both the body and ground - this still does not address the issue that the Earth has no mechanical energy to transfer.

If work is the transfer of energy, how can the Earth do work on a person's body when they land from a fall when the Earth does not possesses any mechanical energy?

 

[Drakkith]

Because the Earth is resisting the motion of the dropped object. It is part of the system that is involved in the work. Does a wheel on a pulley posess any energy? No, but it is integral to the machine that uses the pulleys.

Also realize that it is entirely valid to look at the problem from the frame that the Earth is moving and the person is not. Either way is fine. If a plane falls from the sky to the surface of the Earth that is facing the orbital direction of the earth, then it is entirely correct to think of it as the Earth moving towards the plane.

 

[Naty1]

Let's consider a tennis ball bouning off a wall...the KE at impact causes a minor deformation of a few molecules in the wall, a bit of frictional heat, and the ball is compressed (and heats a bit) and then rebounds back away from the wall. Maybe the wall undergoes a bit of vibration and so KE is dissipated as well that way...

All your example are similar...in the car for example, whatever energy it took to,say, form the final rounded/curved metal shape from the flat sheet has been expended...I don'lt think there is anyway to "measure" that energy in the final product. During formation you could use you original logic...w = fd, some heat is producted,etc...So when it gets crumpled in an accident some more heat is produced, and the lattice formation of the metal is further deformed using some of the "crash" energy...the car also rebounds, moves, and briefly has some KE until frictional forces slow it down producing heat...

 

[AlephZero]

Many people explain that deformation of a car striking a solid object or a body striking the ground as the object or ground doing work on the care or body.

Hmm... I've never heard anybody try to explain the deformation by that argument.

I'm guessing what your problem is, but I think you are confusing two different things, "force" and "work" (or "energy").

If a car strikes a solid object, then the car exerts a force on the object, and the object exerts and equal and opposite force on the car. (That is Newton's third law of motion).

But that is not the same as saying the car does some work on the object, and the object does an equal (or opposite?) amount of work on the car to deform its shape.

Work = force x distance, as you said.

If the solid object doesn't move, then the force the car exerts on the object does no work, because the "distance" = 0.

The forces acting on the car are not just at the point of impact. There are internal forces distributed through all of the car, and those forces do "move" a short distance as the car decelerates and parts of it get bent by the impact. That is what does work on the car. These forces reduce the kinetic energy of the car from what it was before the impact to zero.

According to the law of conservation of energy, the car's kinetic energy has be transformed into something. A small amount of it is converted into vibrations in the air around the car, which you hear as the noise of the car crash. More of it is converted into heat - in other words, the individual atoms that make up the car vibrate a bit faster, which corresponds to a rise in temperature. The rest of it is converted into strain energy "locked up" in the bent parts of the car. If you have learned about the strain energy in a stretched spring, then you can think of the bent parts of the car as like a spring that is stretched and then "locked" into the stretched position. so it can not return to its original shape, and the strain energy inside it can not be released again.

 

[Naty1]

PS: simple questions rarely have simple answers...

This article gives some additional details in the first few paragraphs...I did not read the whole thing...
http://en.wikipedia.org/wiki/Deformation_(engineering )

 

[Rap]

You have to watch where the energy goes. Consider a special case - the Earth and the object are like weights with springs on them. The object starts out with potential energy E, and this gets converted to kinetic energy as the object falls. When the object touches the earth, both the Earth and the object springs compress. Since the Earth is so massive, it returns almost all of that energy to the object, and the object rebounds back into space. During the time the two are in contact, that is the time when work is done on the object. But half the time the object is moving towards the earth, half the time away, so the total distance moved is zero - the work done on the object is zero, and the objects kinetic energy is the same as when it hit (but moving away from the Earth now).

Now take the case when the object is not springy. It hits the earth, transfers energy to the earth, which is then fed back to the object, but this energy is not used to make the object move but to break it. The energy is used to rearrange the objects parts, with friction dissipating the energy returned ultimately as heat. In this case, the object does not bounce, the total distance it moved while it was being subjected to a force from the Earth is not zero, and the work done on it is not zero. The object's kinetic energy is not the same as it was when it hit, it is now zero. A lot of its kinetic energy has been spent rearranging it, and then being dissipated as heat. (some of it has also been spent heating the earth, creating sound waves, etc)

 

[selftaught]

Thanks AlephZero. For the first time in a couple of weeks I feel I'm finally getting somewhere.

I think I've got the concepts of force, work, and energy sorted, but my description of my problem and attempting to put it into technical terms may be suggesting I don't.

My area of interest is injury and injury science defines injury in terms of 'exposure' to energy. Some refer to 'transfer' of energy. It's easy to conceptualise the transfer of KE from a moving bat to a shin, but, with a falling body it's difficult to see any transfer of energy given the Earth does not possesses any mechanical energy (assuming no deformation).

1. W=Fd. I understand the person/car does no work on the ground/object as the d=0 in the latter. Newton's 3rd law. Doesn't the reaction force of the ground/object do work on the person/car and deform them/it? ... which then leads me back to my original dilemma concerning work being defined in terms of energy transfer and that the ground/object does not possesses any mechanical energy.

2. Have not seen the argument concerning the internal forces before. A new avenue of attack. I'm a little confused as to one thing in your explanation: 'those forces do "move" a short distance as the car decelerates' - what are the forces moving? As I understand the concept of work (Fd), the forces are applied to an object or body.

3. Your energy 'locked' in deformation explanation. Just been reading about 'irreversible energy conversions', 'inelastic energy', and plastic deformation. Gerry Carr in Sport Mechanics for Coaches describes strain energy as 'ability to restore themselves back to their original shape after being squashed, etc'. The idea of inelastic energy I just cannot understand at this point. Energy is defined as the capacity to do work. How does the dented body of a car after a crash possesses the ability to apply a force over a distance?

4. I was going to leave this question for another thread, but since I've got your attention, law of conservation of energy, in a fall, gravitationl potential energy is converted to KE, and on impact that KE is converted to sound and heat energy. Assuming the ground does not deform, the person's body deforms so the KE accumulated during the fall is converted to strain energy. But when the deformation exceeds the physiological tolerance levels and say bones are broken, that strain energy is converted to what (given that energy is neither created nor destroyed, but only changes form)? Some will be to sound energy as the bone breaks, some heat; but is there any other transformations or is all the strain energy (or KE) converted only to sound and heat?

Thanks by the way. Cheers

 

[selftaught]

Rap. Thanks. I've seen the retransmission argument before, but I cannot get past the description of work as being the transfer of energy. The ground has no mechanical energy. The object hits the ground. Given a hard surface that does not deform. There is no change in mechanical energy of the ground, therefore, no work is done on the ground and the KE accumulated during the fall has not been transferred to the ground. It has been converted to strain energy in the object (person's body which is the subject of my interest; law of convervation of energy in action) and then the deformation exceeds tolearnce levels and an injury occurs (see my question on what became of the strain energy/KE when the tolerance levels were exceeded in reply to AlephZero post). The ground can apply a force, but given it has no mechanical energy, how can it transfer any.

The retransmission argument is that KE is first transferred to the ground then back to the object/body. What is the process? With a nondeformed impact surface, the ground does not move, it does not deform. and it is the ground so no change in gravitational potential energy. How is the KE transferred, that is work done, on the ground given no change in mechanical energy of the ground? If no change in mechanical energy of the ground and no mechanical energy to start, how can the energy be retransmitted to the object/body?

I'm amazed this issue is not covered in the many dozens of biomechanics and physics books I've researched looking for an answer.

I've just read about a second unnamed work-KE theorum in Benjamin Crowell's book which refers to these situations but does not explain it at all well.

Thanks.

 

[Drakkith]

1. Yes, the force of the Earth causes the object, in this case a car, to crumple and deform. From Wikipdedia: In physics, mechanical energy describes the sum of potential energy and kinetic energy present in the components of a mechanical system. Mechanical energy is the energy associated with the motion or position of an object Think back to the frame where the Earth is moving. In this case the Earth would posess kinetic energy. Or you could say that the Earth and the car are both moving towards each other at whatever the falling velocity of the object is.

2. As the car deforms, its components are being moved in different directions from the compression. The body and frame is bending and flexing and such. Since they are moving this is work. The force is the kinetic energy of the car causing the materials to bend and flex and break.

3. If a car panel is dented, then portions of it have been moved over distance. Therefore work has been done.

4. The strain is being absorbed by the body and the earth. It is being transformed into heat, sound, and strain on the body. When the force of the fall is greater than a certain amount, the body starts to break and get injured. It is still absorbing energy, but its like hitting a pole at 1 MPH in a car compared to hitting another car at 70 MPH, if the energy is too much, then the strain limit has been exceeded and it starts to break.

 

[Drakkith]

Rap. Thanks. I've seen the retransmission argument before, but I cannot get past the description of work as being the transfer of energy. The ground has no mechanical energy. The object hits the ground. Given a hard surface that does not deform. There is no change in mechanical energy of the ground, therefore, no work is done on the ground and the KE accumulated during the fall has not been transferred to the ground. It has been converted to strain energy in the object (person's body which is the subject of my interest; law of convervation of energy in action) and then the deformation exceeds tolearnce levels and an injury occurs (see my question on what became of the strain energy/KE when the tolerance levels were exceeded in reply to AlephZero post). The ground can apply a force, but given it has no mechanical energy, how can it transfer any.

The retransmission argument is that KE is first transferred to the ground then back to the object/body. What is the process? With a nondeformed impact surface, the ground does not move, it does not deform. and it is the ground so no change in gravitational potential energy. How is the KE transferred, that is work done, on the ground given no change in mechanical energy of the ground? If no change in mechanical energy of the ground and no mechanical energy to start, how can the energy be retransmitted to the object/body?

I'm amazed this issue is not covered in the many dozens of biomechanics and physics books I've researched looking for an answer.

I've just read about a second unnamed work-KE theorum in Benjamin Crowell's book which refers to these situations but does not explain it at all well.

Thanks.

Think of an object being dropped onto a trampoline. The springs absorb the energy of the object and then transfer it back when the springs return to their original shape. Now imagine the ground is like a very very stiff spring. It is still absorbing the energy, but instead of taking a second and 2 feet of movement to absorb it, this happens nearly instantly and with nearly 0 movement.

 

[Andy Resnick]

Thanks AlephZero. For the first time in a couple of weeks I feel I'm finally getting somewhere.

I think I've got the concepts of force, work, and energy sorted, but my description of my problem and attempting to put it into technical terms may be suggesting I don't.

My area of interest is injury and injury science defines injury in terms of 'exposure' to energy. Some refer to 'transfer' of energy. It's easy to conceptualise the transfer of KE from a moving bat to a shin, but, with a falling body it's difficult to see any transfer of energy given the Earth does not possesses any mechanical energy (assuming no deformation).

<snip>

3. Your energy 'locked' in deformation explanation. Just been reading about 'irreversible energy conversions', 'inelastic energy', and plastic deformation. Gerry Carr in Sport Mechanics for Coaches describes strain energy as 'ability to restore themselves back to their original shape after being squashed, etc'. The idea of inelastic energy I just cannot understand at this point. Energy is defined as the capacity to do work. How does the dented body of a car after a crash possesses the ability to apply a force over a distance?

4. I was going to leave this question for another thread, but since I've got your attention, law of conservation of energy, in a fall, gravitationl potential energy is converted to KE, and on impact that KE is converted to sound and heat energy. Assuming the ground does not deform, the person's body deforms so the KE accumulated during the fall is converted to strain energy. But when the deformation exceeds the physiological tolerance levels and say bones are broken, that strain energy is converted to what (given that energy is neither created nor destroyed, but only changes form)? Some will be to sound energy as the bone breaks, some heat; but is there any other transformations or is all the strain energy (or KE) converted only to sound and heat?

Thanks by the way. Cheers

You've been getting some great answers, maybe I can add a little.

when someone falls, energy is not transferred from the ground to the person. Instead, what happens is that the potential energy (becasue the person was high up) converts to kinetic energy (the person is now moving), and upon contact, is converted into something else: what form it takes next depends on how contact occurs.

Consider a person jumping off a table onto the floor. Unless they are drunk (or stupid), they will not just fall flat on the ground, but as they hit the ground, they will absorb the impact by using their muscles and joints, ending in some sort of crouched position before standing up and walking away. All of the energy that was made available by falling (converting potential energy into kinetic energy) was absorbed by the person's body by an *elastic* collision: ignoring muscle physiology, the energy was absorbed by stretching out muscles, which act like springs. The muscles return to the nominal lengths, everything is back to the way it was, and no energy was dissipated ('lost') in this case. Now, including muscle physiology, the energy was in fact dissipated by the muscles as they contracted to resist and slow the motion after contact with the floor, and because the person had to expend energy to stand up.

Now consider the drunk who falls off the bar and lands on his shoulder, dislocating it. Now, instead of the energy being absorbed by springs, all the energy went into dislocating the shoulder. And damaging the soft tissue, perhaps tearing ligaments, etc. etc. Now, when the person stands up, their body has been damaged in a way that did not occur above- it has undergone a *plastic* deformation.

But you already know this, as you mention in point 4. What you missed was that breaking/tearing/etc is *also* a conversion of energy. The mechanism is not completely understood, but as stress is applied and an object undergoes strain, that "strain energy" is initially elastic: the material acts like a spring ("elastic deformation'), and will return to the original shape if the stress is removed. Increasing the applied stress, the material then deforms irreversibly ('plastic deformation'), and that permanent deformation is the result of an object absorbing energy- the energy has dissipated, and cannot be recovered to perform work. At some point, fracture occurs, and some of the stress energy is used to create additional surface.

 

[selftaught]

Thanks Andy. I think the most telling comment you made was 'the mechanism is not completely understood', which I can confirm based on a couple of weeks 8 hours a day research on this subject.

Injury science defines injury in terms of exposure to (some refer to transfer of) energy. So, unlike all other descriptions that confuses rather than enlightens the reader, I'm trying to consistently apply and reconcile concepts. Not use this concept to explain this aspect and another to explain another aspect with the apparent discrepancies (for those who do more than skim the material) left unresolved.

I've found the conservation of energy is used to explain the GPE being converted to KE which is then converted to heat and sound when a rock hits an undeformed surface. When we talk about damage (which includes injury) resulting from this impact, we then move away from the conservation of energy (CoE) to talk about absorption, dissipation, and elastic, inelastic, plastic deformation. When the term 'lost' or 'absorbed' is used and reference had been made to CoE where energy can only change form, it raises questions. Energy can't be lost, it can only change form - what form? It can't be absorbed, it can only change form - what form?

The answer given by some is in terms of inelastic energy, energy that cannot be released. Energy is the capacity to do work. How can it be energy if it cannot be used to do work? Seems, to this novice, a bit of 'shoehorning' is going on.

I am stunned what appears to me to be a simple matter is not covered by the many texts that have been written on mechanics and as applied to other disciplines such as biomechanics. That they all do not follow through with the argument, eg. the conservation of energy ends with KE being converted to heat and sound energy, and 'deformation' - this is how Gerry Carr explains the impact of a trampolinist with the floor after making a mistake in her routine, and when this example is used to explain the CoE principle. If energy cannot be created or destroyed, what happened to the trampolinist's KE on impact. Deformation is not energy. Deformation energy also known as strain energy is energy. If the tissues can return to their original shape the KE has been converted into strain/deformation energy. If they cannot, they have resulted in permanent or plast deformation. This is not energy.

You can see I think the original comment of yours I cited at teh beginning is appearing to me to be the most insightful.

Thanks for you help. Adds more to my knoweledge base.

 

[Drakkith]

I am not sure what you are saying Selftaught, so I have to ask, did you understood everything here or do you have a problem with something?

 

[selftaught]

Thanks for the concern Drakkith. Yes I have understood everything that has been said. To a large degree, the responses have reflected my research. No agreement and no definitive explanation.

When explaining conversion of energy (CoE) in a falling body, this principal is used to explain GPE converted to KE during the fall which is converted to heat, sound, and deformation/strain energy upon impact. If the surface does not deform then the body takes all the KE and deforms. But, if the body tissues do not have the ability to reform they do not possesses strain energy. That is, they deform permenantly or rupture. Based on a purely CoE analysis/explanation and that energy cannot be created or destroyed but only change form, what form did the KE or strain energy convert into when the tissues ruptured? Nobody seems to be able to explain this, and different concepts are used once we cannot 'identify' what form the energy transformed into.

Some try to explain the permanent deformation in terms of inelastic energy or other terms to refer to energy that cannot be relesed. Energy is the capacity to do work, that is apply force over a distance, so if energy cannot be released surely it is not energy.

Many describe the deformation/damage of a body when impacting with the ground as being work done by the ground on the body. Many suggest since the ground is not discplced the fallen body does not work on the ground. But W=Fd and Newton's 3rd law; while the falling body may not being doing work on the ground, the reaction force of the ground is doing work on the body. But when work is defined as the transfer of energy, where is the energy that the ground is transferring to the body.

Some have suggested the Earth actually possesses mechanical energy (ME) and that is what is transferred. Others that work is not being done by the ground upon the fallen body but internal forces are responsible for work on the body which results in deformation. Others suggest the minutest undetectable movements of the ground are strain energy from the transfer of KE from the fallen body which then rebounds to transfer the KE back to the fallen body. There appears to be no definitive explanation of how energy is 'transferred' to the fallen body on impact. The explanations are many, they conflict, appear flawed and sometimes appear like exercise in shoehorning. From a force perspective, I have no problem, from a transfer of energy perspective ... well.

And here I was thinking this was a simple question.

 

[Drakkith]

I think you are misunderstanding Selftaught. The replies here are mostly in agreement with each other, some of us just can't explain it well or don't know it detailed enough to really explain it. =)

But, if the body tissues do not have the ability to reform they do not possesses strain energy.

I don't think this is true. The body can withstand stresses up to the point that they exceed a certain level, at which point you start getting permanent damage. I'm not familiar with "Strain Energy" but I think that is what you were asking.

Based on a purely CoE analysis/explanation and that energy cannot be created or destroyed but only change form, what form did the KE or strain energy convert into when the tissues ruptured?

The kinetic energy was released as the rupturing or shattering or whatnot of the body. Specifically the energy is released by breaking connective tissues, blood vessels, and other things. Are you asking what is the exact name of this energy? It's still kinetic energy. Take the spring on a trampoline, materials can resist changing and absorb energy until they reach their breaking point. I can't give you the exact name of what it would convert into, as I don't know.

But W=Fd and Newton's 3rd law; while the falling body may not being doing work on the ground, the reaction force of the ground is doing work on the body. But when work is defined as the transfer of energy, where is the energy that the ground is transferring to the body.

The ground is transferring the bodies own kinetic energy back at it. Honestly, if it helps, you might want to think of the ground as not transferring any energy, so instead the energy can't transfer anywhere else but the body, resulting in injury and such.

Some have suggested the Earth actually possesses mechanical energy (ME) and that is what is transferred. Others that work is not being done by the ground upon the fallen body but internal forces are responsible for work on the body which results in deformation. Others suggest the minutest undetectable movements of the ground are strain energy from the transfer of KE from the fallen body which then rebounds to transfer the KE back to the fallen body. There appears to be no definitive explanation of how energy is 'transferred' to the fallen body on impact. The explanations are many, they conflict, appear flawed and sometimes appear like exercise in shoehorning. From a force perspective, I have no problem, from a transfer of energy perspective ... well.

You don't always get the exact same answers from everyone here, but most are generally about the same thing, just explained in a different way. Not all of us are immersed in physics and science as much as others.

 

[Rap]

The ground has no mechanical energy. The object hits the ground. Given a hard surface that does not deform. There is no change in mechanical energy of the ground, therefore, no work is done on the ground and the KE accumulated during the fall has not been transferred to the ground.

As mentioned above, yes, the ground does deform, its just that it is very small. The Earth MUST apply a force to the object, and as you say, it cannot do this without some deformation. You could say that it does not deform, but then you have to say that the force of the Earth on the object is infinite for an infinitely small amount of time, so that the product F dx is finite.

 

[selftaught]

I think you are misunderstanding Selftaught. The replies here are mostly in agreement with each other, some of us just can't explain it well or don't know it detailed enough to really explain it. =)




I don't think this is true. The body can withstand stresses up to the point that they exceed a certain level, at which point you start getting permanent damage. I'm not familiar with "Strain Energy" but I think that is what you were asking.

Strain energy is one of the three froms of mechanical energy along with gravitational potential energy and kinetic energy. Strain energy refers to the ability of an object to restore itself to its original shape once deformed. Your example of a trampoline is an example of strain energy when it is deformed. That is stored energy which is then converted into kinetic energy when reforming which is transferred to the trampolinist when they bounce upwards. There is no question associated with stresses and failure.

The kinetic energy was released as the rupturing or shattering or whatnot of the body. Specifically the energy is released by breaking connective tissues, blood vessels, and other things. Are you asking what is the exact name of this energy? It's still kinetic energy. Take the spring on a trampoline, materials can resist changing and absorb energy until they reach their breaking point. I can't give you the exact name of what it would convert into, as I don't know.

Exactly! What did the KE energy that caused the deformation that resulted in damage convert into? It has to convert into some form of energy according to the law of conservation of energy. The CoE doesn't allow for 'release' or 'absorption', it only allows for transformation from one form to another. Even if we look at the term 'released', what does that actually mean? It is released into what? The effect is damage to the body's tissues, no doubt, but 'released'? Released into another form of energy according to CoE.

It is still kinetic energy unless you identify something that is moving. KE of a bat is applied to a bone which initially deforms. This slight deformation prior to failure is a transfer of KE to strain energy as the bone reforms. If the bone breaks, there is no strain energy in the bone as it does not have the ability to restore itself to its original shape. I can't see that the broken bone possesses kinetic energy.

The ground is transferring the bodies own kinetic energy back at it. Honestly, if it helps, you might want to think of the ground as not transferring any energy, so instead the energy can't transfer anywhere else but the body, resulting in injury and such.

Unfortunatly it doesn't help. I'm writing a book applying injury science to the tactics and techniques of combatives (no matter however they are described). I am also drawn like light to black hole to flawed arguments. Work is described as the transfer of energy and as Fd. With Newton's 3rd law that is the 'F' of the ground reaction force and the deformation of the falling body that is 'd', the ground appears to be doing work on the body when it impacts. Given the explanation of work as being the transfer of energy, back to this quandry, what energy is being transferred to the body given the body did not transfer any energy to the ground because the displacement of the ground was zero.

You don't always get the exact same answers from everyone here, but most are generally about the same thing, just explained in a different way. Not all of us are immersed in physics and science as much as others.

I've written to a number of professors of biomechanics and have not received any responses as yet. The explanations provided here, while appreciated, appear to confirm what I have read to date in my research - that there is on definitive explanation concerning the issues I've raised.

Cheers :)

 

[selftaught]

As mentioned above, yes, the ground does deform, its just that it is very small. The Earth MUST apply a force to the object, and as you say, it cannot do this without some deformation. You could say that it does not deform, but then you have to say that the force of the Earth on the object is infinite for an infinitely small amount of time, so that the product F dx is finite.

OK. Now that I can see, albeit, I have not come across in the multitude of books I've referred to on this subject. ... more research now to find an authoritative source for this explanation now. You see, I'm writing a book concerning injury science and as I'm no expert I'm supporting all my arguments with authoritative sources.

Thanks RAP.

 

[selftaught]

Drakkith. Sorry about my posting. I'm new here and didn't know how to do the quoting thing. I've come here after exhausting all other resources. Cheers

 

[Dale]

The explanations provided here, while appreciated, appear to confirm what I have read to date in my research - that there is on definitive explanation concerning the issues I've raised.

To me it seems like you got some good definitive explanations. Which specific issue still lacks a definitive explanation?

 

[selftaught]

DaleSpam ... my exploration of these issues is really crystalising them in my mind, so thank you for the opportunity for presenting them in hopefully a more succinct form.

1. Conservation of Energy (CoE) - energy is neither created nor destroyed, it can only change form.
2. Principle of CoE is often explained using a falling object as an example.
3. Gravitational potential energy (GPE) is transformed to kinetic energy (KE) during the descent.
4. Upon impact GPE = 0, KE = 0 of the object after KE was maximum just before impact.
5. If the impact surface does not deform and neither does the object then 100% of the KE has been transformed into sound and heat energy.
6. If the impact surface does not deform and the object does, some of the KE was transformed into deformation.
7. If that deformation returns to its original shape, the KE was transformed into strain energy (SE; also referred to as deformation energy or elastic energy).
8. If the deformation goes beyond its tolerance limits, damage results which means the object/body cannot return to its original shape, which means the SE has been transformed into another form of energy. 'Absorbed', 'dissipated', 'lost', are all used to explain the effect, but, it does not explain it in terms of the law of CoE which states that energy can only be transfromed from one form to another.
9. What form of energy does this SE take when tolerance limits are exceeded and the material/tissues cannot restore themselves to their original shape, ie. when damage results?

The work issue is hopefully resolved based on a recent posting, however, subject to finding authoritative support. With regards to this issue, you can see I'm not alone in being confused as to the energy mechanism in the injury process. The World Health Organisation refers to 'transfer of energy or prohibits the transfer of energy'. Whiting and Zernicke refer to two principles in the injury process: conservation of energy and transfer of energy. They appear to be both trying to explain injuries occurring when energy is transferred and where it appears not to be transferred.

This situation involves the concept of work.

1. W=Fd
2. McGinnis: work is 'the means by which energy is transferred from one object or system to another'.
3. W = change in KE + GPE + SE
4. Someone hitting another person resulting in deformation beyond tolerance levels is the transfer of KE. No problems (apart from above). Work is being done by the person hitting on the person being hit.
5. Many suggest the injury resulting from a fall is because of the work done on the person's body by the impact surface.
6. The ground has no ME: KE + GPE + SE = 0.
7. Was any energy transferred from the falling body on impact?
9. Many suggest (with the exception of a recent post) that no deformation occurs on a hard surface therefore there is no change in ME, therefore no work is done by the body on the ground, therefore the ME of the ground remains 0.
10. No ME means nothing to transfer therefore the ground cannot do work on the person's body. We need another principle to explain the injuries; maybe CoE.
11. Unfortunatley Newton's 3rd law and W=Fd suggests work is done on the faller's body. The reaction force is applied to the body which deforms it.
12. But where did the energy come from to be transferred when work is conceptualised in terms of the transfer of energy?
13. Some suggest 'retransmission'. This relies on energy being first transferred to the impact surface, ie. work being done by the body on the impact surface, and then the impact surface transferring the energy to the body, ie. the impact surface doing work on the body.
14. If the impact surface SE = 0 upon impact, this retransmission argument would appear to fail, even though the W = Fd argument still holds, albeit we cannot identify where the energy came from to transfer to the person's body resulting in deformation and injury.

I'm now on the hunt for an authoritative source that will support the infintesimal increase in SE upon impact resulting in a transfer of KE from the faller's body to the ground which then rebounds transforming SE into KE and transferring it to the faller's body by doing work on it. If anyone can point me in the direction of a book that would provide this support, very much appreciated :).

 

[Dale]

Surely you do not think that ALL of these 23 points lack definitive explanation. For instance, the first number 1, conservation of energy, is clearly as definitive as anything can possibly be. Energy is conserved.

So I ask again. Which issues are the ones that lack definitive explanation?

 

[selftaught]

Surely you do not think that ALL of these 23 points lack definitive explanation. For instance, the first number 1, conservation of energy, is clearly as definitive as anything can possibly be. Energy is conserved.

So I ask again. Which issues are the ones that lack definitive explanation?

DaleSpam. Thanks for your post. Regarding the specific issue that you raised. If you think the conservation of energy (CoE) is clearly as definitive as anything can possible be, please explain what form of energy the kinetic energy (KE) accumulated during a fall is transformed into when an injury or damage occurs when a body or an object impacts with the ground and the ground does not deform. The CoE states that energy cannot be created or destroyed, it can only be transformed into another FORM of energy. The CoE does not refer to the release, dissipation, or deformation of energy. It states one form of energy transformed or converted into another.

I've just read a book on Conservation Laws by Benjamin Crowell. He refers to an example of a comet impacting Jupiter and 100% of the KE being transformed into heat and sound energy. It is a very simplistic example and of course ignores deformation of the surface of the planet. ... and in this case, if the deformation is not deformation/strain/elastic energy, that is, if the deformation does not reform to its previous shape, than that KE that caused the deformation has been transformed into another form of energy. What form of energy has that KE been transformed into? THAT question has not been answered here, nor is it answered in any of the multiple texts I've referred to, nor has it been answered tonight by the profession engineer who resides in the same apartment block as I do.

That is one issue that lacks an explanation, let alone a definitive explanation.

The issue of reconciling work, the transfer of energy, and injury or damage resulting from impact from a fall has only one potential explanation in the posts to date. Unfortunately, Crowell tends to cast doubt on it. Again, no definitive support from an authoritative source can be found to explain what energy, or where it came from, is being transferred to the impacting body to cause deformation, let alone damage. The common response is, as my engineer source tonight immediately suggested and which was evidenced in the posts, can't you answer the question from a force perspective. You CAN. No question. BUT, from an energy perspective, there are problems. The ground has no mechanical energy, and given, as Crowell suggests, no work is done/energy transferred by the impacting body with the ground, the ground has NO mechanical energy to transfer to the body causing deformation of any description.

 

[Dale]

If you think the conservation of energy (CoE) is clearly as definitive as anything can possible be, please explain what form of energy the kinetic energy (KE) accumulated during a fall is transformed into when an injury or damage occurs when a body or an object impacts with the ground and the ground does not deform.

In the end, heat and strain. There is a transient change of some of the energy into mechanical waves, but in relatively short order those go to heat. Which is the next point that lacks a definitive explanation?

Btw, you really need to learn to be more concise.

 

[Andy Resnick]

Energy is the capacity to do work. How can it be energy if it cannot be used to do work? Seems, to this novice, a bit of 'shoehorning' is going on.

Hang on- not all energy can be used to perform work. Energy takes on many different forms, and can be interconverted between forms. The process is not 100% efficient (from the laws of thermodynamics), and so some energy is permanently 'lost', in the sense that it cannot be used to perform work.

 

[selftaught]

In the end, heat and strain. There is a transient change of some of the energy into mechanical waves, but in relatively short order those go to heat. Which is the next point that lacks a definitive explanation?

Btw, you really need to learn to be more concise.

Most refer to heat and sound energy. Strain energy refers to the ability of an object/body to restore itself to back to its original shape after being deformed. If the deformation goes beyond the limits of its ability to restore itself back to its original shape, eg. it is damaged, by definition it does not possesses strain energy. Hence, the KE was not transformed into strain energy. You're only left with heat and sound energy then. Based on that logic, you'd be arguing that in a fall which results in damage and no deformation of the impact surface the kinetic energy accumulated during the fall is converted into heat and sound energy only. Therefore, injuries are caused by KE being converted into heat and sound energy. If that is your argument, then you're probably the first to articulate that argument. And I could not argue against you as I've not found anything that argues (or supports) that proposition.

Re concise - I wonder what explanations would have been provided if I simply (as I have) asked, what form of energy is the KE transformed into when a body/object impacts a nondeforming surface? As has been seen by the responses, that probably would not have helped.

But thanks for your post. It's all helping in building an appreciation that this issue is not well considered nor understood.

 

[selftaught]

Hang on- not all energy can be used to perform work. Energy takes on many different forms, and can be interconverted between forms. The process is not 100% efficient (from the laws of thermodynamics), and so some energy is permanently 'lost', in the sense that it cannot be used to perform work.

I am focussed on mechanical energy primarily. In mechanics, the definition of energy is the capacity to do work. Given the conservation of energy, energy cannot be created nor destroyed. I would have thought 'permanetly lost' would equate to being destroyed. I understand the not 100% efficient idea, but the energy that is 'lost' transforms into another form of energy that then may be used to do work. If I'm incorrect I'd appreciate being corrected, but with a reconciliation with the idea that energy cannot be created nor destoryed and its definition as the capacity to do work.

Cheers :)

 

[Rap]

Thanks for the concern Drakkith. Yes I have understood everything that has been said. To a large degree, the responses have reflected my research. No agreement and no definitive explanation.

Ok, if you want to get serious, let's get serious. The best way to understand what is going on is to start with simple examples and build up to more complicated ones. A semi-elastic collision between two bodies is the most complicated, so that comes last.

First, you consider an elastic collision. Think of a small mass with a small spring to the right of it, moving with velocity V to the right. That's "the object". Then, to the right of it is a large mass with a large spring to the left of it. That's the Earth, its not moving. When the object and the Earth first touch, both springs start to compress. The Earth exerts a force on the object, compressing its spring and slowing it down, the object exerts an equal force on the Earth, compressing its spring and making the Earth move to the right. The Earth is so massive, that we can say it hardly moves at all. After a while, the object comes to rest because of the force of the Earth spring pushing back on it. Now both springs are in their maximum deflection. Some of the original kinetic energy is in the object spring, some of it in the Earth spring, the total of both is the original kinetic energy of the object when it first touched the Earth. The Earth and the object keep pushing on each other, and the object starts to move to the left, away from the Earth. Both springs start to decompress. When the object gets to the position where it first touched the Earth, both springs are back to their original unstressed length, and all of the energy that was contained in the springs has now been transferred to the kinetic energy of the object as it moves to the left, away from the Earth. To summarize, before the object touched the Earth, all the energy was its kinetic energy. During the collision, that kinetic energy was converted to the potential energy of the springs. Part of the kinetic energy went to potential energy of the Earth spring, part went to the kinetic energy of the object spring. Then, when the object started to move away from the Earth, the potential energy of both springs was transferred to the object, until it had regained all of the original energy which is now its kinetic energy, except it is moving to the left, not the right. As far as work is concerned, the total distance either object moved while the forces were in effect is zero, while the force at any point was the same going in as when going out. The work done on the object is zero - its energy is unchanged. The work done on the Earth is zero - it is still motionless.

Now let's do an inelastic collision. In this case, the entire kinetic energy of the colliding object gets converted to heat, and it sticks to the Earth, not by gravity, but because the collision is inelastic. We can think of an inelastic body ("object" or "Earth" or both) as a very damped spring, it gets compressed, but the energy that was formerly held by the compression of the spring is now converted into heat energy, and the spring stays compressed. The object has been deformed (or "injured"). Also, there is a partially elastic body, where its spring gets compressed and bounces back, but does not bounce back to its normal position. Part of the energy going into it is converted to heat, part is stored as potential energy of compression. There are many ways to have an inelastic collision. You can have the object be inelastic while the Earth is elastic or partially elastic, the object can be elastic or partially elastic, while the Earth is inelastic, or you can have both be inelastic. If both are partially elastic, the object will bounce off the Earth, but not as energetically as if the collision were elastic.

Lets say the object is inelastic, the Earth is elastic. Then, just as before, the object spring and the Earth spring get compressed until the object stops. The energy that would have been kept in the object spring as potential energy is now converted to heat, and the object spring stays compressed. The Earth spring expands, compressing the object spring even more, all of which gets converted to heat in the object. When the Earth spring has expanded to its normal position, the object and the Earth are motionless, and all of the original energy has been converted to heat contained in the object. The object spring is compressed (injured), and contains no potential energy. As far as work is concerned, the force on the object as it moves in is not the same as the force on the object as it moves out. Although the total distance moved while the force is in effect is zero, the forces are not the same at a particular point going in as going out. This means the integral of F dx is not zero, the work done on the object is not zero, but is equal to the original kinetic energy, all of which has been converted to heat.

Now let's say the object is elastic, the Earth is inelastic. The same thing happens, but in reverse. The object and Earth wind up motionless, but now all the kinetic energy of the object has been converted into heat that is contained in the Earth, and the Earth spring is permanently compressed, with no deformation or "injury" to the object. Work has been done on the Earth, not the object, and its energy has increased. All of the original kinetic energy of the object is now contained in the Earth as heat, and the Earth is deformed.

Notice that deformation is associated with conversion to heat. An elastic body does not deform and does not acquire heat in a collision.

Now we have to consider the real case. If the collision is not completely inelastic, the object will bounce off the Earth after the collision. In the real case, there is a little bounce, but let's say we can ignore that. Now we can say the collision is inelastic. But is the object inelastic while the Earth is elastic or partially elastic? Maybe the object is elastic or partially elastic while the Earth is inelastic? Maybe both are inelastic. All of these possibilities will yield an inelastic collision.

So now we have a more complicated set of possibilities. I think we can assume that the object is inelastic, since it gets very deformed. I'm not sure, but I think we might say that the Earth is inelastic as well. In this case, part of the initial kinetic energy goes into heating the object, part goes into heating the Earth. How much goes where depends on the strength of the object spring and the Earth spring. Since the Earth is much "harder" than the object, I think we can say that most of the kinetic energy is converted to heat in the object and the Earth ultimately absorbs only a small fraction of the energy. I might be wrong on this.

The process of the kinetic energy being absorbed by a real object is complicated. The simple idea of a spring does not cover it all. There are pieces sliding past each other, creating friction heat, there are damped springs being compressed, there is viscous effects of fluids being mechanically stirred which converts to heat. There is "flattening" of the object, but that probably doesn't account for a lot of the energy. There are sound waves generated in the object, which are eventually dissipated into heat. Also, there is a third object here - the atmosphere. Some of the energy gets converted to atmospheric sound which radiates away, but that probably doesn't account for a lot of the energy. The bottom line is that a good portion of the object's kinetic energy is passed to the Earth, which is then passed back to the object. Most of the kinetic energy winds up in the object as the form of heat, with the object being quite deformed.

If you go over the 23 points, I think that the above argument shows that:

5 is wrong, heat is generally associated with deformation, except for viscous stirring of fluids.

6 is wrong, there is no "energy of (permanent) deformation". Permanent deformation occurs because energy has been converted to heat.

7 is right, but incomplete - the strain energy is then released, to either impart kinetic energy, or converted to heat.

8 is correct but not covered by the above argument - This is a case where you could say the spring breaks. When this happens, pieces of the object are suddenly moved around, colliding elastically and semi-elastically with each other, creating heat and internal sound waves, all of which finally wind up as heat, with the resulting deformation being evidence of that process.

9+6 is incorrect. Again, and again, and again, the ground acquires energy from the object upon impact. You have to understand that this is true. Just because it seems hard does not mean it acquires no energy. If you have a very strong spring, it need hardly move at all in order to absorb a lot of energy.

9+7 - YES

9+9 wrong
9+10 wrong
9+12 Again, from the impacting object
9+13 Again, correct
9+14 Again, SE of the Earth is not zero, it acquires energy from the impacting object.

 

[Dale]

Most refer to heat and sound energy.

That is fine. I mentioned mechanical waves (sound) as a transient form of energy that quickly decays into heat. I prefer the term mechanical waves as it includes compression waves that are outside of the audio frequency as well as shear waves which are not generally associated with sound and probably cause the majority of the soft tissue damage in a fall. But as long as everyone understands that we are talking about a generalized "sound energy" then that is OK.

Strain energy refers to the ability of an object/body to restore itself to back to its original shape after being deformed. If the deformation goes beyond the limits of its ability to restore itself back to its original shape, eg. it is damaged, by definition it does not possesses strain energy. Hence, the KE was not transformed into strain energy.

Some of the body's KE does remain as elastic strain energy even after severe plastic deformations. Consider an undamaged body simply at rest on the ground compared to one in free fall. The falling body is unstrained and the body on the ground is elastically strained. Similarly for a damaged body at rest on the ground.

It's all helping in building an appreciation that this issue is not well considered nor understood.

If by not well understood you mean by you, then I agree. If you mean by the field in general, then you are simply wrong. The equations of motion have been around for about 3 centuries and are well known and validated. Additionally modern computational techniques make realistic modeling of viscoelastic materials quite feasible.

 

[Mapes]

Based on that logic, you'd be arguing that in a fall which results in damage and no deformation of the impact surface the kinetic energy accumulated during the fall is converted into heat and sound energy only. Therefore, injuries are caused by KE being converted into heat and sound energy. If that is your argument, then you're probably the first to articulate that argument. And I could not argue against you as I've not found anything that argues (or supports) that proposition.

Also note that when an object fractures, energy is stored via the creation of additional surface area. When a crystal permanently deforms, energy is stored via the creation of defects (dislocations) in the crystal. It's not all just heat.

 

[selftaught]

Rap - thanks for the comprehensive response. With regards to the responses to my individual points:

1. '5 is wrong, heat is generally associated with deformation, except for viscous stirring of fluids.' Many texts use the example of a stone falling and on impact with the ground niether the stone nor the ground deforms and all the KE accumulated in the fall is transformed to sound and heat energy.

2. Your # 6, 7, and 8 appear to be supporting my evolving assumption as to the transformation of KE when the tolerance levels of an object/body are exceeded, that is, it is converted into heat and sound energy only. If so, that is a simple answer to a simple question that has not been forthcoming from any of my research and I cannot find authoritative support for this proposition unfortunately. I wish I could because I could then move on.

3. I'd love to gain support for your 9+6 as it would provide the answer I'm looking for, however, I cannot find support for it. As posted tonight, Benjamin Crowell's Conservation Laws provides examples where this is not the case. I'm still searching for authoritative support for this proposition.

4. Much of the rest of your responses are reflected in my #3 above. It's not that I'm arguing against you, it's that I cannot find authoritative support for the argument.

Your lengthy disertation, no offence intended, reflects what I've found in mechanics, physics, and biomechanics. Heaps of concepts, but they don't follow through with just one concept. For instance, when I research and discuss the work issue, most people cannot answer the question in energy terms and instead answer it in force terms. If injury is defined in energy terms, as it is in injury science, you want everything to be related to energy ... if you want it to be understood, respected, and used by non-academics. Also for instance, many suggest you can look at problems in terms of momentum and impulse, or, energy and work. I know that's true, but, if injury is defined in terms of energy 'exposure' (not transfer; see previous post) then you'd need to reconcile momentum with energy ... if its to be intelligible to those not versed in the many different concepts in these fields.

Just a simple thing like you're suggesting, energy is transferred from the impacting body to an apparently undeformed impact surface is then transferred to the impacting body resulting in deformation and work being performed by the impacting surface on the impacting body - after dozens and dozens of texts in mechanics, physics, and biomechnics, not one suggests this is the case. I'm not saying its not the case, I'm just saying they don't follow through with there explanations to explain this process.

It is a frustrating process (a) attempting to understand these simple questions, and (b) not finding any assistance in the numerous texts I've referred to.