# Homework Help: Elastic collision between two billiard balls

1. Jul 26, 2013

### phyme814

1. The problem statement, all variables and given/known data
Two balls collide on a frictionless surface. Mass m1 = .4 kg and Mass m2 = .3 kg . The initial velocity of m1 is 3 m/s. While m2 is at rest. After the collision the velocity of m1 is 1.5 m/s, θ1 above the horizontal. While m2 gains an unknown velocity of v2f, θ2 below the horizontal.
Find v2f, θ1, and θ2

2. Relevant equations

Conservation of kinetic energy
Conservation of momentum

3. The attempt at a solution
I can find v2f, but I am having trouble with finding the angles.

2. Jul 26, 2013

### Staff: Mentor

Show what you did.

3. Jul 26, 2013

### Staff: Mentor

I presume you used conservation of energy to solve for v2f. Now use conservation of momentum to solve for the angles. (You'll get two equations and two unknowns.)

4. Jul 26, 2013

### phyme814

Yepp I used conservation of kinetic energy.
I was using the conservation of momentum equation for the unknowns of the angles but I was running into trouble because I would get sinθ1 and sinθ2 in one equation and cosθ1 and cosθ2 in the other equation.

5. Jul 26, 2013

### Staff: Mentor

Sine and cosine can be related by a trig identity.