One ball has a mass of m1 = 0.250 kg and an initial velocity of 5.00 m/s. The other has a mass of m2=0.800 kg and is initially at rest. No external forces act on the balls. What are the velocities of the balls after the collision? Elastic Collision. On the book I have this formula. STEP 1# 1/2 m1vf1^2 + 1/2 m2Vf2^2 = 1/2 m1V01^2 + 0 (I am confused for the 1/2 at the beginning and why is squares?) WHAT???