Why does the work done by a weightlifter result in a net energy loss?

[david316]

Hello,

I'm getting a little confused and would appreciate some clarification.

If a weight lifter lifts a weight of 2500N a distance of 2m, the work done by the weight lifter's force on the weight is 5000J. The work done by the weight force due to gravity is -5000J. The weight lifter then slowly lowers the weight to the ground so the work done by the weight is +5000J and the work done by the weight lifter is -5000J.

So the total work done by the weight lifter is 0J. It does not seem reasonable to me that the weight lifter regains the energy. How much energy does the weight lifter use?

Thanks

 

[anuttarasammyak]

You are right to get total 0J. Why do you feel uneasiness on it ?

 

[david316]

I guess the way I think about it is that when the weight lifter lifts the weight he uses energy to do so. So the weight lifter has used 5000J of energy in lifting the weight. Presumably from the energy stores in his body. i.e. he burns calories. But when he lowers the weight he does not get the energy back. So to say he has used no energy seems incorrect?

 

[russ_watters]

But when he lowers the weight he does not get the energy back. So to say he has used no energy seems incorrect?

It's because our muscles aren't springs and we expend energy just applying a force, regardless of the motion. In other words, we keep expending more energy, we don't get it back.

 

[david316]

So is it fair to say that when he lifts the weight he does 5000J of work plus additional energy of his muscles "applying" the force (i.e. contractions etc similar to using energy when a human pushes against a wall). If we assume this additional energy is negligible I can say he used 5000J lifting the weight. When he lowers the weight he will be using some energy due to his muscles contracting but if I assume this is negligible can I say he does 0J of work lowering the weight or does he do -5000J of work lowering the weight?

 

[david316]

I guess ultimately I am trying to relate the work done as a product of force times distance to the energy used by the weight lifter. For example if I weigh 1000N and do ten pushup of 0.1m each can I say I would have used at least 1000J of energy as 1000N * 0.1m * 10 = 1000J.

 

[phinds]

I guess ultimately I am trying to relate the work done as a product of force times distance to the energy used by the weight lifter.

That's a false/meaningless equivalence as has already been shown.

 

[david316]

Forgive my ignorance but when the weight lifter lifts the weight and does 5000J of work on the weight where does the energy come from to do that 5000J work?

 

[phinds]

Forgive my ignorance but when the weight lifter lifts the weight and does 5000J of work on the weight where does the energy come from to do that 5000J work?

Where do you think it comes from? Clearly you understand that it doesn't just appear like magic, so what could be the mechanism?

 

[david316]

Ok sure...but I'm struggling to understand why the relationship between work and energy use is a meaningless equivalence.

If the weight lifter does 5000J work he must use at least 5000J of energy in lifting the weight. When he lowers the weight he must use greater than 0J of energy in lowering the weight. He doesn't get the energy from lifting the weight back when he lowers it. So why do we say the work the weightlifter did when lifting and lowering the weight was zero when the net energy he used was greater than zero?

Surely there must be some relationship between work done by the human body and energy used by the human body? If there wasn't what the point in calculating the work the human body does?

 

[david316]

Also, thank you for you time. I do appreciate it.

 

[Lnewqban]

Ok sure...but I'm struggling to understand why the relationship between work and energy use is a meaningless equivalence.

If the weight lifter does 5000J work he must use at least 5000J of energy in lifting the weight. When he lowers the weight he must use greater than 0J of energy in lowering the weight. He doesn't get the energy from lifting the weight back when he lowers it. So why do we say the work the weightlifter did when lifting and lowering the weight was zero when the net energy he used was greater than zero?

Surely there must be some relationship between work done by the human body and energy used by the human body? If there wasn't what the point in calculating the work the human body does?

The weightlifter also does some muscular work when lowering the weight, if he tries to guide and slow down its return to the floor.
There is no way for the muscles to regain the work back.

This is not different from your city pump station using energy to lift and move water into houses, just to be spilled down the drains.

The fuel and engine of an airplane gives energy to the air around the plane, so it can fly high.
The same plane will surely return to the ground if that engine stops.

Gravity is persistent and frequently wins.

A hydroelectric station uses great deal of the energy used by the Sun to evaporate water, lift it and accumulate it in a lake or dam.
That is something that our weightlifter can't do himself.
He could attach a mechanism to the weigh in such a way that it does some work while returning to ground level (like pumping some water, or moving an electric generator, for example).

 

[david316]

Ok...I was mixing up work with energy used. I now realize they are not the same thing. Just because the weight lifter does zero work which is clear as the net transfer of energy to the mass is zero that does not mean he uses zero energy in lifting and lowering the weight.

 

[david316]

Would there be anyway to work out how much energy the weight lifter uses when lowering the weight?

 

[phinds]

Would there be anyway to work out how much energy the weight lifter uses when lowering the weight?

Do you think it would be the same for all lifters? The same, for example, between a 28 year old professional weight lifter lifting 50 lbs and a 16 year old weakling just barely managing to lift the same 50 lbs?

 

[david316]

I don't know if a strong person has greater efficiency lifting weights than a weak person so don't know.

When lowering the weight if the lifter applies 250N to oppose the 2500N of the weight and assume ideal efficiency of 1.0 how much energy does the lifter use when lowering the weight.

 

[Ibix]

Do you know the old claim that "according to aerodynamics, bumblebees can't fly"? It's not true, of course, but if you naively apply fixed-wing human sized aircraft aerodynamics to a bumblebee, you will run into trouble. Similarly, trying to analyse a complex system like a human in terms of simple work-done calculations will drive you crazy.

There must be a way to deduce the energy use in the situation you describe because we are not exempt from the laws of physics. It's easy enough experimentally - you just measure heat output, which is where the energy goes eventually. But you will need a much more complex model of a human than "a black box that applies a force" to do it theoretically, I'm afraid.

 

[david316]

Do you know the old claim that "according to aerodynamics, bumblebees can't fly"? It's not true, of course, but if you naively apply fixed-wing human sized aircraft aerodynamics to a bumblebee, you will run into trouble. Similarly, trying to analyse a complex system like a human in terms of simple work-done calculations will drive you crazy.

There must be a way to deduce the energy use in the situation you describe because we are not exempt from the laws of physics. It's easy enough experimentally - you just measure heat output, which is where the energy goes eventually. But you will need a much more complex model of a human than "a black box that applies a force" to do it theoretically, I'm afraid.

Thanks.

 

[anuttarasammyak]

But when he lowers the weight he does not get the energy back. So to say he has used no energy seems incorrect?

Let me take a more mechanical and simple example of helicopter or drone. When it goes up, it consumes fuel or cell energy. After it has reached the expected height and is hovering, it is consuming energy also. When it is going down slowly to the ground, it consumes energy again. It should keep rotating wing anytime.
In the mechanism of helicopter flight generation of momentum, i.e. applying force, requires energy consumption as well as in the mechanism of human muscle.　I have read that starfish muscle would not consume energy when it sustain a weight in the air because of a hooking mechanism which is specific to its species.

 

[david316]

There must be a way to deduce the energy use in the situation you describe because we are not exempt from the laws of physics. It's easy enough experimentally - you just measure heat output, which is where the energy goes eventually. But you will need a much more complex model of a human than "a black box that applies a force" to do it theoretically, I'm afraid.

I'm slowly getting there but would you be able to elaborate? In particular when the weight lifter lowers the weight as I understand it, the force of the weight will do positive work on the weightlifter. So 5000J of energy is transferred from the weight to the weightlifter. Is this correct? If it is correct where does this energy go?

 

[david316]

I guess it's not going into kinetic energy as the weight lifter so it presumably has to go into potential energy (elastic potential energy of the muscles I guess) and heat?

 

[anuttarasammyak]

You might do experiment by yourself. Please lye on your back and raise your legs slowly to upright position and let them down slowly to the ground. I assume you can experience what happens in your body by getting potential energy of legs into your body.

 

[Ibix]

Is this correct? If it is correct where does this energy go?

Correct. The energy is dissipated as heat, via the lifter's muscles.

If the lifter was a Toyota Prius, he could use his regenerative braking to recharge his batteries as he brakes the fall of the weight. Unfortunately, humans don't work that way and we waste the work done on us.

 

[A.T.]

I guess ultimately I am trying to relate the work done as a product of force times distance to the energy used by the weight lifter.

The relationship is:

work_done = efficiency * energy_used

Where efficiency can be negative, meaning that you are consuming energy to collect even more energy via negative work, and then both energy inputs are dissipated as heat.

This is what muscles do, when they are stretched, while being tensioned. But it also happens when you brake a car using the engine in reverse gear and with partially engaged clutch (not recomended).

I guess it's not going into kinetic energy as the weight lifter so it presumably has to go into potential energy (elastic potential energy of the muscles I guess) and heat?

There is only very limited energy storage in the tendons, which is primarily useful for dynamic movements, like in locomotion.

 

[rudransh verma]

Hello,

I'm getting a little confused and would appreciate some clarification.

If a weight lifter lifts a weight of 2500N a distance of 2m, the work done by the weight lifter's force on the weight is 5000J. The work done by the weight force due to gravity is -5000J. The weight lifter then slowly lowers the weight to the ground so the work done by the weight is +5000J and the work done by the weight lifter is -5000J.

So the total work done by the weight lifter is 0J. It does not seem reasonable to me that the weight lifter regains the energy. How much energy does the weight lifter use?

Thanks

Because the force we use to lift is not a conservative force. There is no potential energy stored in our body when we lift. Its like the work done by frictional force. The frictional force changes the kinetic energy to thermal energy but it can't reverse it back to kinetic energy. Energy is always lost. $\Delta E_{mechanical}\neq0$.
We can understand this with something like dead things gain energy by mechanical forces but when that mechanical energy is applied on us we will always loose energy. Like beating a person will make him completely exausted.
I hope this helps.
(I have drawn a diagram where the spring is a part of the body.)