# Convservation of Mech. Energy/Momentum

1. Dec 12, 2006

### AznBoi

I have a question about the conservation of momentum and energy in collision problems. What type of situations (elastic/inelastic) can you use the equations to solve for either masses or velocities??

Am I correct?::

Elastic collision: You can use both conservation of momentum + mech. energy to solve for the variables because everything is conserved.

Inelastic collision: You can use only conservation of momentum because some energy is wasted as heat or wasted from friction.

Can someone give me a better explaination or correct me if I'm wrong? Thanks!

2. Dec 13, 2006

### OlderDan

Not only can you use energy conservation for elastic collisions, you must use it to solve the problem. An algebraic manipulation can be done to produce a velocity difference equation that can be used instead of the quadratic energy equations for head on collsiions. If a collision is perfectly inelastic, there is only one final velocity for both objects involved.

Some problems fit between these two categories and give a coefficient of restitution that expresses the final velocity difference as a fraction of the initial velocity difference.

3. Dec 13, 2006

### enricfemi

Yes,you are both right.
conservation of enegy can only be used in elastic collision.
And in the inelastic collision the coefficient is
e=(U1-U2)/(v2-v1)
U is former velocity