# Kinetic energy of rotation

1. Apr 21, 2010

### jwu

I just have a kind of conceptual question. I am doing a problem. It's related to a ball rolling from a inclined plane without slipping. The answer uses the Conservation of Mechanical Energy to attack that problem. But isn't true that Conservation of Mechanical Energy can only be used when there isn't any nonconservative force? The force of friction between the plane and the ball is a nonconservative force, isn't it? So why can it use the Conservation of Mechanical Energy to do that problem?
Overall, my problem is simply. Why is Mechanical Energy conserved in "rolling without slipping on inclined plane", with the nonconservative force of friction?
It's my first post in this forum. thank you.

2. Apr 21, 2010

### zachzach

My guess: Energy is NOT conserved. Instead of Ei = Ef you have Ei = Ef + Work done by friction.

3. Apr 21, 2010

### jwu

But the prolem is that it's the answer on the Priceton AP physics book. It stated that "we will attack this problem using Conservation of Energy." Then it comes out something like:
Ki+Ui=Kf+Uf
0+Mgh=(1/2Mv^2+1/2Iw^2)+0
any ideas? Thank you anyway!!!

4. Apr 21, 2010

### zachzach

So your saying friction is needed for the object to roll but if this is so then why is energy conserved? This has to mean that the frictional force that causes the rolling does no work. But Why not? If the sphere was sliding there would be work done.

Last edited: Apr 21, 2010
5. Apr 21, 2010

### jwu

So you mean the Mechanical energy is conserved because the frictional force that causes the rolling dose no work?

6. Apr 21, 2010

### zachzach

http://cnx.org/content/m14391/latest/ Scroll down. The explanation is that the work done by the static friction in rotation is equal and opposite to the work done in translation thus canceling out.

7. Apr 21, 2010

### jwu

You are amazing!!! This really helps a lot! Just enlightening!!! How can I add you as a forum friend?