# Homework Help: Energy Transfer

1. Jan 17, 2008

### Gear300

I sort of need help conceptualizing how this energy system would work.

There is a boulder on the ground and a person is attempting to move it by pushing it. However, because of the static friction, the boulder does not move and ultimately, no work is done. However, there must have been some energy transfer since after some time, the person feels tired. If the energy transfer were written as:
Wp - Wf = Wnet (Wp - work done by person, Wf, work done by static friction, and Wnet - net work), then it could be stated as 0 - 0 = 0. I'm assuming this implies that the energy transfer was not mechanical. The problem here is that, if the person doesn't move the boulder, then there is no work done, and since static friction doesn't accelerate the boulder, no work is done by that either. So then, what was the energy transfer going on here? And also, does static friction influence energy systems (does it process energy into heat, etc...?).

2. Jan 17, 2008

### mgb_phys

You have to be carefull with the physics notion of work and human work.
If you didn't move anything then friction did no work, you could have transferred energy into compressing the boulder, the soles of you shoes and the ground. This would have been converted into heat when you stopped pushing.

3. Jan 17, 2008

### Tom Mattson

Staff Emeritus
This is where you get into biochemical stuff. When you exert yourself physically, your muscles are flexed. It takes energy to do this, even if you don't move the rock anywhere. Now think: where must that energy come from?

4. Jan 17, 2008

### Gear300

I see...so the energy goes into deforming the rock and shoes, flexing the muscles, and so forth. This energy is then converted to heat. The source of this energy should be biochemical or energy that the person pushing originally retained.

5. Jan 18, 2008

### Tom Mattson

Staff Emeritus
Right--From food. So eat your vegetables!