# Homework Help: Conservation and angular velocity

1. Oct 30, 2011

### shaqtus

1. The problem statement, all variables and given/known data
I need help with this question: http://img804.imageshack.us/img804/2278/unledsbg.jpg [Broken]

For a, I got omega = 18.63 rad/s by using methods of conservation of energy. Can someone tell me if I did this right? If not, please help me out! To be honest, I thought I had to use conservation of momentum for this since it involves a collision, but its equations dont involve angular velocity.

2. Relevant equations

Conservation of energy/momentum

3. The attempt at a solution

Last edited by a moderator: May 5, 2017
2. Oct 30, 2011

### Staff: Mentor

Energy is not conserved. But angular momentum is.

3. Oct 30, 2011

### shaqtus

So does that mean I find v by means of conservation of momentum, and then use omega = v / r to find the answer? The reason I'm confused is because at the note at the bottom of the question, it says treat the door as a rod rotating about its end, which is a hint to use Inertia = (1/3)ML^2. Conservation of energy, not momentum, has inertia in its equation.

4. Oct 30, 2011

### Staff: Mentor

Conservation of angular momentum will involve the moment of inertia.