# 2D Collision Problem: Given 2 masses, a velocity, and an angle, find another angle and a velocity

I was given the mass and velocity of an object, and I know that it is traveling in a straight line along the x-axis (so I suppose I've been given a second angle). It collides with an object with known mass at rest. The first object rebounds at a given angle. What is the velocity (magnitude and direction) of the second object after the collision? You can't assume that it's an elastic collision.

I started with two component forms of the conservation of momentum equation, and reduced to three variables by replacing $Ax'$ with $A'cos(\theta A')$, and so on for each component. But now I have three variables and two equations! I've racked my brain, and I can't think of what to use as a third equation. What should I do?

• Conservation of kinetic energy as the collision is elastic. – Farcher Nov 1 '16 at 23:18
• I said at the end of the first paragraph that you can't assume it's elastic. – RothX Nov 1 '16 at 23:26
• The problem has no unique solution if you don't know that the collision is elastic, you would need not just the angle of the first object, but also its velocity. You can tell because you know the initial and final momentum (same as initial), but by letting the velocity of the first object be a variable, you get a range of momentum and energy for the second object. That's not a unique solution. – Ken G Nov 1 '16 at 23:50
• @KenG Well, I have the initial velocity, but not the final. Is that what you mean? – RothX Nov 1 '16 at 23:53
• @KenG That should probably be posted as an answer – David Z Nov 1 '16 at 23:55