# Homework Help: Elastic collision in 2d

1. Nov 12, 2007

### fliinghier

1. The problem statement, all variables and given/known data
there are two masses, the smaller sitting still, and the larger with 5 times the mass of the smaller hits it going 12 m/s. the smaller rebounds at an 80 degree angle from the direction of the original mass. the collision is elastic. find the speed of both objects and the angle of the larger one after the collision.

2. Relevant equations
1/2mv^2 (KE, which is conserved)
mv (momentum, which is conserved)

3. The attempt at a solution

so far i have tried using sin and cos of theta and 80 degrees to find equivalent equations using momentum(5V2sin(theta)=V1sin(80) and 60=5V2cos(theta)+V1cos(80)) and then i tried to plug variables into the KE equation or solve the equations simultaneously.

2. Nov 12, 2007

### Staff: Mentor

You are on the right track. Try this: Use the momentum equations to find V2 in terms of V1. Then plug that into the KE equation. (Hint: Take advantage of the trig identity $\sin^2\theta + \cos^2\theta = 1$.)

3. Nov 12, 2007

### fliinghier

after trying this way again i got stuck (again) when i reached the following:

720=(V1^2)(1+(.970/(sin(theta))^2))

4. Nov 12, 2007

### Staff: Mentor

Use the hint I gave to eliminate theta before plugging into the KE equation.

5. Nov 12, 2007

### fliinghier

thanks i think i got it now.