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Let's say we do a push up. If asked to compute the work, it would be Mass(lifted) x G(gravity) x height moved. I get that.

What confuses me is how/what force does the work?

Part of me feels it would be the ground, because as we push down, the ground pushes back forcing us up. However, this makes no sense as the ground has no energy to give and the distance at the point of application is zero.

So that must mean the work is done by the muscles. But how so? Really yes the muscles start pushing down, which causes the ground to push back, making the muscles internally then push up and move the body, hence we say the person/muscles did the work.

I never thought of it before. If I think of it as the muscles pushing the body up, obviously this is not an issue. But I realized for the muscles to push up, they must push the ground too, which doesn't move.

Sorry if this is repetitive, I am really confused. Thank you for the help

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Essentially the force from the ground just acts as an energy converter (here from chemical to mechanical) and in due process you neither gain nor lose any amount of energy (assuming that dissipation in the form of heat is negligible). A similar case is of walking where frictional force converts your chemical energy into your kinetic energy. During the whole process the ground neither gains nor loses the energy (assuming that there is negligible change in earths rotatory motion).

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Your muscles perform the work and provide the force to raise your body. Your body rises because you are pressing against the Earth, which is effectively immovable owing to its enormous mass. If you consider a push-up in more detail you will see that it is the muscles in your upper arm and chest which contract to create turning forces at your elbow and shoulder. Those are the forces doing the work. The Earth is simply resisting the push of your hands.

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  • $\begingroup$ Marco, so would you say my 2nd to last paragraph is essentially correct and saying the same thing? The muscles of the body, apply force to the body because the earth is non movable. The reaction from the ground is what allows the muscles to move the body in response to the earth not moving. So in the end, work is done by the person. Agree? $\endgroup$ Nov 18 '19 at 16:40
  • $\begingroup$ Yes, that's pretty much it. $\endgroup$ Nov 18 '19 at 17:45
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So that must mean the work is done by the muscles. But how so? Is it I am confusing this? Really yes the muscles start pushing down, which causes the ground to push back, making the muscles internally then push up and move the body, hence we say the person/muscles did the work.

When talking about work you need to be relatively careful to specify what is the "system". Here there are at least two obvious and useful possibilities. One is to consider the whole body as the system, and the other is to consider the arm/chest muscles to be a separate system from the skeleton and the rest of the body.

System = Whole body

In this case, there is no work done. The internal chemical PE is simply converted to gravitational PE, thermal energy, and perhaps a bit of KE if the person is showing off or just in a hurry. Since all of that is an internal conversion of energy there is no work done on the system. This is consistent with the fact that you already recognized that the ground does not move.

System = Arm/chest muscles

In this case, there is work done. The internal chemical PE of the muscles is converted to thermal energy and work. There is a strong force on the tendons and joints which move through a distance thereby doing mechanical work. That mechanical work from the muscles system is used by the skeleton/body to increase gravitational PE and perhaps KE. In the force is large and the distance is small, the function of the skeleton and even the ground is basically to act as a complicated lever system which translates that small distance and large force into a large distance and small force.

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    $\begingroup$ I was wondering if I was looking at wrong and clearly I've been mixing systems..I have to recognize mostly I've been taught to look at it from the arm/chest system. But if I look at the body as a whole, then all those forces are internal, all the work internally transforms/converts the energy Really the ground reaction force is helping to make that happen, but it does no work. There was an example in my physics book explaining how external reaction forces help transform the energy internally of a system. Now that makes more sense $\endgroup$ Nov 18 '19 at 20:30

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