Matt is gliding on his skateboard at 4.00 m/s. He suddenly jumps backward off the skateboard, kicking the skateboard forward at 8.00 m/s. Matt's mass is 50 kg and the skateboard's mass is 7 kg. How fast is Matt going as his feet hit the ground? This what I did: P_f = P_i v_f(50kg + 7kg) = 50kg*4m/s + 7kg*4m/s v_f = 4.49m/s ?