# Homework Help: Conservation of Energy

1. Nov 18, 2007

### feedmeister

Hi all,

I'm just working some practice problems, and I can't think of any examples for these two questions. Any help?

1. The problem statement, all variables and given/known data
Give a specific example of a situation in which:

$$W_{ext}\rightarrow E_{th}$$ with $$\Delta K=0,$$ and $$\Delta U=0.$$

$$U\rightarrow E_{th}$$ with $$\Delta K=0,$$ and $$W_{ext}=0.$$

2. Relevant equations
K-f + U_f + Delta E_th = K_i + U_i + W_ext

3. The attempt at a solution

I'm trying to find examples of these situations... any help?

Thanks!

2. Nov 18, 2007

### dynamicsolo

Can you think of something where you would be pushing it without having its kinetic energy or potential energy change at all, so that all your work just goes into, say, heating the surfaces in contact?

How about something where the potential energy being released only goes into heating up contact surfaces, without having the kinetic energy change and without something actively pushing it along?

3. Nov 18, 2007

### feedmeister

Hey,

Thanks for the response. I know what the question is asking, but I can't think of situations in which they occur.

Thanks

4. Nov 18, 2007

### dynamicsolo

Well, for the first one, how about pushing a table across a carpet or a rough floor so that it moves at only a constant velocity?

The second one would involve something falling through a rough passage or through a viscous fluid at terminal velocity, so that the change in potential energy is dissipated as heating or disturbance in the fluid, without the object gaining speed.

5. Nov 18, 2007

### feedmeister

Wow, thanks dynamicsolo, I've been trying to think of scenarios in which those cases would happen, but I never thought to have constant velocity and have it fall through something.