1. The problem statement, all variables and given/known data Mass 1 has a mass of 5kg and is traveling with an initial velocity of 20 m/s at a 45 degree angle. (Starting in Quadrant II and heading toward the origin.) Mass 2 has a mass of 6kg and is traveling with an initial velocity of 15 m/s at a 45 degree angle. (Starting in Quadrant III and heading toward the origin.) The final velocity of mass 2 is known and it is 25 m/s after the collision. The collision is completely elastic. I need to find the velocity of mass 1 after collision, and the final angles of both masses. 2. Relevant equations I don't know all of the relevant equations. But I think I need to use: ∑mvx(before)=∑mvx(after) ∑mvy(before)=∑mvy(after) 3. The attempt at a solution First I broke the initial velocity of mass 1 down into its x and y components. Since the initial angle is 45°, the x and y velocities are the same. (20m/s)*cos(45°)≈14.14214 Then I broke the initial velocity of mass 2 down into its x and y components. (15m/s)*cos(45°)≈10.60660 Next, I thought that since I knew the final velocity of mass 2, I should be able to calculate its angle using: ∑mvx(before)=∑mvx(after) [(15m/s)*cos(45°)]/(25m/s)=(3√2)/2 Then cos-1([3√2)/2])≈64.896° I'm not sure if these steps are correct up to this point ... but I went on to try to calculate the final velocity of mass 1 using: m1v1^2+m2v2^2=m1v'1^2+m2v'2^2 With that equation I calculated that the final velocity of mass 1 is ≈-3.1299m/s I'm extremely confused on this problem and feel like everything I'm doing is wrong.