A 1.2 kg ball moving due east at 40 m/s strikes a stationary 6.0 kg object. The 1.2 kg ball rebounds to the west at 25 m/s. What is the speed of the 6.0 kg object after the collision
v_2' = v_1 + v_1' So v_2' = 40 + -25 We have set east to be + and west - Which gives us 15 m/s. So thats how fast the 6 kg object is going. This is true for an elastic collision.