Like what IS energy? relative to force

[Andrew Mason]

There is a force applied to the body, this is the force of the rope, that keeps the body at constant distance(radius) from the axis.
At the other end of the rope there is the same magnitude force with opposite direction. These 2 forces are responsible for the tension in the rope.
There is no tension with only one force.

As Newton says

There is a net force on the rotating body. We know this from Newton's first law.

Yes, Newton's third law applies. Let's make this a simple case of gravitational orbit, such as the moon and Earth orbiting each other, (to make it as simple as possible we will ignore other bodies such as the sun in this example). They both rotate about a common centre of rotation. The Earth exerts a gravitational force on the moon and the moon exerts an equal and opposite gravitational force on the Earth (Newton's Third Law). Since there is only one force acting on the moon and one on the earth, the force on each body is unbalanced (ie the gravitational forces on each are net forces). This means that both bodies must be accelerating toward each other, and this is exactly what occurs.

If you think that there is no force acting on the axis, make it from fragile material and it will break when body rotates fast.

That occurs because the tether is not strong enough to supply the needed force. When it breaks away, there is no force acting on the body at all.

Malverin: your questions are good ones for someone who has not yet formally studied physics. I expect that many scholars before Galileo and Newton asked similar questions. Physics provides answers that has changed the way we look at the world. This is why physics is such an important subject.

AM

 

[Dale]

It is inaccurate yes.
When resultant force is not zero is more accurate.
Because there will be movement or/and deformation.

Even that is incorrect. Uniform circular motion is a counterexample where there is a non zero resultant force on a system with no transfer of energy to or from the system.

Again, a system in physics need not be isolated.

 

[Malverin]

That occurs because the tether is not strong enough to supply the needed force. When it breaks away, there is no force acting on the body at all.AM

I say that the axis will break because a force is applied to it.

And you say it will break, but there is no force, because there is only 1 force and it is applied to the rotating body.
So then axis is broken from nothing. Interesting suggestion

 

[alodia]

Malverin: your questions are good ones for someone who has not yet formally studied physics. I expect that many scholars before Galileo and Newton asked similar questions. Physics provides answers that has changed the way we look at the world. This is why physics is such an important subject.

AM

and this is precisely the problem with education in general today. anyone who needs the education clearly haven't had 'formal studied' before. even if they had, any new knowledge to be learned makes their prior knowledge 'informal' so please it is better to use examples and explain from the 'learner's perspective of knowing NOTHING' rather than the 'scholar's perspective of already having developed one's own system of understanding and just regurgitating what the scholar knows'

 

[Andrew Mason]

I say that the axis will break because a force is applied to it.

The rope will break because the tension in the rope just exceeds, at least for a brief moment, the rope's tensile strength.

And you say it will break, but there is no force, because there is only 1 force and it is applied to the rotating body.
So then axis is broken from nothing. Interesting suggestion

I did not say there was no force on the rotating body before the rope broke. I said there was no force on the body after the rope broke.

AM

 

[Andrew Mason]

and this is precisely the problem with education in general today. anyone who needs the education clearly haven't had 'formal studied' before. even if they had, any new knowledge to be learned makes their prior knowledge 'informal' so please it is better to use examples and explain from the 'learner's perspective of knowing NOTHING' rather than the 'scholar's perspective of already having developed one's own system of understanding and just regurgitating what the scholar knows'

By 'formal' study I meant one that is designed to teach the concepts, details and skills that are needed in order to gain a proper understanding of the subject. One can engage in a formal study of something on their own. But it is usually better if the subject is taught by someone who has that understanding.

AM

 

[Malverin]

The rope will break because the tension in the rope just exceeds, at least for a brief moment, the rope's tensile strength.

I did not say there was no force on the rotating body before the rope broke. I said there was no force on the body after the rope broke.

AM

I have never said that the rope will break! We assume it is strong enough.

I said the axle will break (bend) , but you are just ignoring that fact.

If you don't think it will, you can try and see it for yourself .

And so, what about the axle forces and its breaking(bending) ?

 

[mic*]

and this is precisely the problem with education in general today. anyone who needs the education clearly haven't had 'formal studied' before. even if they had, any new knowledge to be learned makes their prior knowledge 'informal' so please it is better to use examples and explain from the 'learner's perspective of knowing NOTHING' rather than the 'scholar's perspective of already having developed one's own system of understanding and just regurgitating what the scholar knows'

With all due respect alodia, this is a big statement to make considering you haven't made the effort to use capital letters at the start of each sentence. Your grasp of writing tends to suggest you would at least have been taught that much.

AM your response to the above comment was pretty well put in my opinion. However I think you misinterpreted Malverin; "If you think that there is no force acting on the axis, make it from fragile material and it will break when body rotates fast." I would interpret that as referring to the axel.

This is all getting pretty off topic though.

 

[Andrew Mason]

AM your response to the above comment was pretty well put in my opinion. However I think you misinterpreted Malverin; "If you think that there is no force acting on the axis, make it from fragile material and it will break when body rotates fast." I would interpret that as referring to the axel.

It does not really matter if the rope breaks or if the thing it is tied to (the axis - which I assume is some kind of pole mounted on the earth) breaks. The centripetal force required to keep the body rotating exceeds the tensile strength of the material that the body is attached to. At that point the force ends and the body prescribes a straight line path with uniform motion.

I am not sure what Malverin's point is. My original point was that there is no energy being transferred from or to a body that is experiencing uniform circular motion (ie. the application of the central force to the body does not transfer energy). The force has to act through a displacement and since the force is always perpendicular to the change in displacement there is no work being done and no energy transferred.

AM

 

[Dale]

My original point was that there is no energy being transferred from or to a body that is experiencing uniform circular motion by the central force.

I agree. Malverin doesn't seem to recognize that his statement regarding energy and force was wrong. Hopefully the OP wasn't too confused by the resulting tangent.

 

[Malverin]

It does not really matter if the rope breaks or if the thing it is tied to (the axis - which I assume is some kind of pole mounted on the earth) breaks. The centripetal force required to keep the body rotating exceeds the tensile strength of the material that the body is attached to. At that point the force ends and the body prescribes a straight line path with uniform motion.

I am not sure what Malverin's point is. My original point was that there is no energy being transferred from or to a body that is experiencing uniform circular motion (ie. the application of the central force to the body does not transfer energy). The force has to act through a displacement and since the force is always perpendicular to the change in displacement there is no work being done and no energy transferred.

AM

To have a rotation you need at least 3 elements =>

rotating body, another body(axle) and restrictive connection between them(gravitational force. electromagnetic force, rope...)

Without any of these 3 elements, there will be no rotation.
So taking only one of these elements separately is not correct.
And for the 3 elements rusultant force is zero.

Taking only rotating body separately is the same, as when we have 2 people pulling a rope and we take only one of them separately.
He will experience unbalanced force and we could say he is accelerating towards the other man.
The force is not zero. We have an acceleration, but there is no energy transfer.

 

[Andrew Mason]

To have a rotation you need at least 3 elements =>

rotating body, another body(axle) and restrictive connection between them(gravitational force. electromagnetic force, rope...)

Without any of these 3 elements, there will be no rotation.
So taking only one of these elements separately is not correct.
And for the 3 elements rusultant force is zero.

Taking only rotating body separately is the same, as when we have 2 people pulling a rope and we take only one of them separately.
He will experience unbalanced force and we could say he is accelerating towards the other man.
The force is not zero. We have an acceleration, but there is no energy transfer.

Malverin: Are you stating this or asking whether it is correct? Either way, none of this is correct.

In your first example, the resultant force is NOT zero. There is a net force on each body. Consider two bodies in gravitational orbit about their centre of mass. If they were not rotating they would crash into each other. The rotation does not cancel the force that each exerts on the other!

In your second example, there is energy being transferred from the person's muscles to kinetic energy of the person.You appear to be very interested in physics. But you need to read some good physics texts first rather than assume that you can figure it out yourself. You at least have to listen to what others who have studied physics tell you before you assert things. You have to thoroughly understand Newton's laws. You are not applying the laws of motion correctly.

AM

 

[Malverin]

Malverin: Are you stating this or asking whether it is correct? Either way, none of this is correct.

In your first example, the resultant force is NOT zero. There is a net force on each body. Consider two bodies in gravitational orbit about their centre of mass. If they were not rotating they would crash into each other. The rotation does not cancel the force that each exerts on the other!

In your second example, there is energy being transferred from the person's muscles to kinetic energy of the person.You appear to be very interested in physics. But you need to read some good physics texts first rather than assume that you can figure it out yourself. You at least have to listen to what others who have studied physics tell you before you assert things. You have to thoroughly understand Newton's laws. You are not applying the laws of motion correctly.

AM

OK
I see we have to make it visual

Here

Fb => the force applied to the body (centripetal force)

Fr => the force applied to the rope

Fax => the force applied to the axle

So how exactly you are calculating resultant force here and you get non zero result?

 

[Drakkith]

OK
I see we have to make it visual

Here

Fb => the force applied to the body (centripetal force)

Fr => the force applied to the rope

Fax => the force applied to the axle

So how exactly you are calculating resultant force here and you get non zero result?

The issue isn't whether all the forces for the system add up to zero, it's about the force on each object. Take the ball on a tether example. The ball has a force pulling it inward, given by the tether. The ball also exerts an equal amount of force the opposite direction on the tether. But that's it. There are no other forces, so the ball is being pulled inward by the tether with a non-zero net force acting on it.

 

[Malverin]

The issue isn't whether all the forces for the system add up to zero, it's about the force on each object. Take the ball on a tether example. The ball has a force pulling it inward, given by the tether. The ball also exerts an equal amount of force the opposite direction on the tether. But that's it. There are no other forces, so the ball is being pulled inward by the tether with a non-zero net force acting on it.

I have agreed with that at the beginning of discussion.
As I said, there is no rotation without the other 2 elements
So the result what you get when you take only a part of this system is... partial.
Adding/substracting energy to/from one of the elements, changes the energy of the whole system.
So taking only one element and ignoring the others is not correct.

 

[Dale]

So the result what you get when you take only a part of this system is... partial.
Adding/substracting energy to/from one of the elements, changes the energy of the whole system.
So taking only one element and ignoring the others is not correct.

For the third or fourth time, there is no requirement in physics that a system must be isolated. This argument is fallacious.

In physics you are allowed to define the system as you wish. You can, in fact, take only one element and ignore others. It is correct. The laws of physics work correctly for the one element system as well as the whole isolated system.

For a single body in a two body central force problem there is a net force with no energy transfer. Your original statement is false in that respect and your arguments supporting it are fallacious. Please stop.

 

[Andrew Mason]

OK
So how exactly you are calculating resultant force here and you get non zero result?

There is no outward force on the rotating body. If your diagram is showing such an outward force it is wrong. There is only the tension of the rope on the body. This tension accelerates the body toward the centre of rotation. It is called centripetal acceleration.

AM

 

[Malverin]

For the third or fourth time, there is no requirement in physics that a system must be isolated. This argument is fallacious.

In physics you are allowed to define the system as you wish. You can, in fact, take only one element and ignore others. It is correct. The laws of physics work correctly for the one element system as well as the whole isolated system.

For a single body in a two body central force problem there is a net force with no energy transfer. Your original statement is false in that respect and your arguments supporting it are fallacious. Please stop.

That is written in the books

http://en.wikipedia.org/wiki/Rotating_reference_frame

Newton's law in the rotating frame then becomes

Treat the fictitious forces like real forces, and pretend you are in an inertial frame.

— Louis N. Hand, Janet D. Finch Analytical Mechanics, p. 267

Obviously, a rotating frame of reference is a case of a non-inertial frame. Thus the particle in addition to the real force is acted upon by a fictitious force...The particle will move according to Newton's second law of motion if the total force acting on it is taken as the sum of the real and fictitious forces.

— HS Hans & SP Pui: Mechanics; p. 341

This equation has exactly the form of Newton's second law, except that in addition to F, the sum of all forces identified in the inertial frame, there is an extra term on the right...This means we can continue to use Newton's second law in the noninertial frame provided we agree that in the noninertial frame we must add an extra force-like term, often called the inertial force.

— John R. Taylor: Classical Mechanics; p. 328

I said what I have to say.

 

[Dale]

Completely irrelevant. None of the discussion above has involved non-inertial frames.

You made a mistake, plain and simple. But for some reason, instead of simply saying "oops" and moving on you want to double down with irrelevancies and fallacies.

The application of a force does not imply a transfer of energy, nor does the application of a net force.

 

[Malverin]

Completely irrelevant. None of the discussion above has involved non-inertial frames.

You made a mistake, plain and simple. But for some reason, instead of simply saying "oops" and moving on you want to double down with irrelevancies and fallacies.

The application of a force does not imply a transfer of energy, nor does the application of a net force.

http://en.wikipedia.org/wiki/Work_(physics)

Cartesian coordinates

dWx = Fx * dX

dWy = Fy * dY

dWz = Fz * dZ

Polar coordinates

dW_radial = F_radial * dR

dW_tangential = F_tangential *dθ

Indexes show that the change in energy is due to movement/deformation along the coordinate axes

I leave the comments to you.

 

[mic*]

Polar coordinates

dW_radial = F_radial * dR

dW_tangential = F_tangential *dθ [/B]

The centripetal force being discussed is the radial force or F_radial above, and in that instance dR = 0 so dW_radial = 0

Do you concur with that Malverin?

 

[Dale]

http://en.wikipedia.org/wiki/Work_(physics)

Note that if F is perpendicular to v then F.v=0 even if both F and v are non-zero. Therefore you can have a force and movement without a transfer of energy. Your own citation directly contradicts your claim.

 

[Malverin]

The centripetal force being discussed is the radial force or F_radial above, and in that instance dR = 0 so dW_radial = 0

Do you concur with that Malverin?

If there is not movement/deformation , there is no work done.

That is well known.
Discussion about orbiting, was pointed to forces, not displacements.

 

[Malverin]

Note that if F is perpendicular to v then F.v=0 even if both F and v are non-zero. Therefore you can have a force and movement without a transfer of energy. Your own citation directly contradicts your claim.

Speaking about acting force, I always meant the component of the force parallel to the movement/deformation.
I thought that is clear.

 

[Dale]

Speaking about acting force, I always meant the component of the force parallel to the movement/deformation.
I thought that is clear.

It was never clear to me.

So then, do you agree that the original statement was incorrect as written (i.e. without any discussion about components and movement)?

When you apply a Force, you transfer Energy

 

[mic*]

Speaking about acting force, I always meant the component of the force parallel to the movement/deformation.
I thought that is clear.

There is a force that is radial. Everyone is agreeing on that.

There is no radial movement, as per my last post. You agreed. This is movement parallel to the force.

There is only motion perpendicular to the force, as per Dale's last post. (EDIT: 2nd last post)

Are you just being argumentative for kicks?

 

[Malverin]

It was never clear to me.

So then, do you agree that the original statement was incorrect as written (i.e. without any discussion about components and movement)?

Yes it is inaccurate. I have agreed with that

Malverin #35 Dec9-13, 08:02 PM
P: 43
Quote Quote by DaleSpam View Post
Note that, as you say, there remain 2 forces despite the fact that energy is no longer being transfered. Therefore your original statement "When you apply a Force, you transfer Energy" is clearly wrong.
It is inaccurate yes.
When resultant force is not zero is more accurate.
Because there will be movement or/and deformation.

 

[Malverin]

There is a force that is radial. Everyone is agreeing on that.

There is no radial movement, as per my last post. You agreed. This is movement parallel to the force.

There is only motion perpendicular to the force, as per Dale's last post. (EDIT: 2nd last post)

Are you just being argumentative for kicks?

I said. Discussion about orbiting was not about displacements but forces.

If resultant force is zero, displacement doesn't matter, right?

 

[Dale]

Yes it is inaccurate. I have agreed with that

Even if you say "resultant force" instead of "force" it is still wrong. Do you agree? Your previous statement did not fix the problem.

Furthermore, changing it to "resultant force" introduces new problems since you can have a 0 resultant force with a transfer of energy. For example, if a vehicle is going at a constant velocity on a level straight road then the propulsive force from the road is opposed to the viscous drag force from the air. These two forces yield a 0 resultant force, but energy is being transferred from the vehicle to the air. Note that this scenario involves forces parallel to the direction of motion.

 

[Malverin]

Even if you say "resultant force" instead of "force" it is still wrong. Do you agree? Your previous statement did not fix the problem.

Furthermore, changing it to "resultant force" introduces new problems since you can have a 0 resultant force with a transfer of energy. For example, if a vehicle is going at a constant velocity on a level straight road then the propulsive force from the road is opposed to the viscous drag force from the air. These two forces yield a 0 resultant force, but energy is being transferred from the vehicle to the air. Note that this scenario involves forces parallel to the direction of motion.

Yes it is so. Friction and drag take away energy when forces are in equilibrium.

 

[Andrew Mason]

Yes it is so. Friction and drag take away energy when forces are in equilibrium.

Things have gotten a bit off topic. But you seem to still think that a body in gravitational orbit around another body does not experience a net force. I am not sure why you think this but it is wrong. There is no force other than the force of gravity and it is only in one direction (ie. toward the other body).

AM

 

[Malverin]

Things have gotten a bit off topic. But you seem to still think that a body in gravitational orbit around another body does not experience a net force. I am not sure why you think this but it is wrong. There is no force other than the force of gravity and it is only in one direction (ie. toward the other body).

AM

I have attached a drawing. You can see the forces there.

 

[Bandersnatch]

I have attached a drawing. You can see the forces there.

In your picture(which by the way lacks the force keeping the axle in place), the only force acting on the body is the force provided by the tension of the rope(F_b in your pic). F_r acts on the rope itself, not on the body, and is what keeps the rope taut.

The reaction forces are applied at the same point, but act in opposite directions, so on different bodies. Otherwise you could argue that it's impossible to kick a ball, since the leg-ball system has got two equal and opposite forces exactly cancelling each other at the point of contact.


Back to the picture. You could have zero net forces acting on the body, if you chose to use a rotating reference frame. In which case you'd have to add centrifugal force to the drawing, exactly matching F_b in magnitude.
In an inertial reference frame, the mere fact of there being a rotation makes it clear that there has to be an unbalanced force accelerating the body.

Note that if you'd do what others suggested and simplify things by having the force of gravity acting between two bodies rather than using a rope, it would remove the confusing bits while producing the same results.

 

[russ_watters]

I have attached a drawing. You can see the forces there.

Much of the problem here is that you are misusing words. The term "resultant force" does not apply in this thread. You mean "net force". But you are still misusing it: net force is a force that causes acceleration. But you are describing equal and opposite fore pairs in that context, when the reality is that force pairs ALWAYS sum to zero, regardless of motion or acceleration.

What is frustrating here is that it isn't clear if you are purposely being evasive or are just very confused. That's why the thread hasn't been locked yet. Either way, you need to try harder to listen and learn rather than trying to teach or demonstrate your knowledge.

 

[russ_watters]

Speaking about acting force, I always meant the component of the force parallel to the movement/deformation.
I thought that is clear.

Given that the discussion was about circular motion, where the force is perpendicular to displacement, that is highly dubious.