# Homework Help: Elastic collision

1. Nov 23, 2006

### jezelee

Other than finding the intial xy coordinates and knowing gravity as a factor I am not sure where to go with this problem as the formulas I have deal with final angles and not initial. The problem follows:

Two spheres of mass 1kg & 1.5Kg hang @ rest at the ends of strings that are both 1.5m long and attached at the same point on the ceiling. The lighter sphere is pulled aside so that it makes an angle of 60 degrees w/ vertical. The lighter sphere is then released and the two spheres collide elastically. When they rebound, what is the largest angle with respect to the vertical that the string holding the lighter sphere makes?

Help would be greatly appreciated. Thanks.

2. Nov 23, 2006

### KoGs

Remember both momentum and energy is conserved. So initial (total) momentum = final (total) momentum. Likewise, initial (total) energy = final (total) energy.

3. Nov 23, 2006

### turdferguson

A good tip is that the change in height in a pendulum swing is L-Lcos(theta). Draw out a picture so you can see this relationship and then store it in your memory, it comes in handy a lot

As for this question, find the initial velocity before the collision and use that to calculate the final velocity after the collision (KE conserved). Once you know how much kinetic energy it has afterward, find the new maximum height