1. The problem statement, all variables and given/known data The setup is there are two pucks (m1, m2) on an air table and they undergo elastic collision while m2 is stationary. I am asked to find the angle between the two velocities but I can't figure out how without being giving at least one angle. Variables: m1 = 28.0 g = 0.28 kg m2 = 102 g = 0.102 kg v1 = 0.785 m/s v2 = ? 2. Relevant equations Ki = (1/2)m1v1i2 Kf = Ki Kf = (1/2)m1v1f2 + (1/2)m2v2f2 = Ki 3. The attempt at a solution First I went about finding what v2 was by using Kf = Ki (1/2)(0.028)(0.785)2 + (1/2)(0.102)v2f2 = (1/2)(0.028)(1)2 (0.008) + (1/2)(0.102)v2f2 = (0.014) v2f2 = (0.117) v2f = 0.343 m/s Now this is the part I'm stuck, since they want me to find the angle between v1 and v2. If they had given me at least an angle for m1 I could use [0 = m1v1fsin[tex]\theta[/tex] - m2v2fsin[tex]\theta[/tex]] but I have no angles to work with, and I'm not sure how I should arrange the equations to find the angle. If I were to guess I would say that they angle would be 90o since m2 was stationary, but I'm not sure how to prove that and I don't think it applies to this case since I have two different masses. If anyone could help me understand this problem, that would be great!