B Newton's Third Law Question -- Does the Earth Do Work on Me?

[paulb203]

TL;DR Summary

Does the Earth 'do' anything when it pushes up on me?

When I do a press-up I consciously push down on the ground, my muscles tense, I breathe harder, I sweat, etc. When the Earth pushes back up on me (Newton's 3rd Law) is it 'doing anything', or does it push back up on me merely by virtue of it being there?

 

[russ_watters]

It's the latter. The Earth is not a spectacularly inefficient machine like the human body is. It doesn't have to "do anything" but be in the way to push you up.

 

[A.T.]

Newton's Third Law Question -- Does the Earth Do Work on Me?​

When I do a press-up I consciously push down on the ground, my muscles tense, I breathe harder, I sweat, etc. When the Earth pushes back up on me (Newton's 3rd Law) is it 'doing anything', or does it push back up on me merely by virtue of it being there?

Force is not work. Force is related to momentum transfer, which doesn't imply energy transfer.

 

[jbriggs444]

The mechanical work done by the Earth on the bottom surface of your shoes when you jump is approximately zero. This is because the bottom surface of your shoes does not move while the force from the Earth is acting.

If you look for the source of the mechanical work that is done, you will find that it originates in your muscles. Obviously.

If you look at the forces from each muscle on each of its attachment points and multiply each force by the parallel motion of the associated attachment point, the total will be a positive number. That is the mechanical work done by the muscles.

One can multiply the upward force of the Earth on one's body by the upward motion of the body's center of mass and arrive at a similar number. But it is hard to credit the Earth with "doing" this [center of mass] work when it was just sitting there passively the whole time. Better to give credit where credit is due.