Problem and Given Information I must determine the velocity of a hardball after it has been thrown into a box of paper and then I measure that the distance the box moved back. The Law of Conservation of Momentum (and other physics principles) will be employed to do this. When the ball is caught in the box, the ball/box system will initially have the same momentum. Then friction (the external force) will eventually reduce the velocity, and hence, the momentum of the box is 0. mBox = 2.6 Kg mBall = 0.142 Kg Fk = 7N dist. = .57 m Equations Used [itex]f_k = \mu_k N[/itex] [itex]Fn = mg[/itex] All kinetic motion equations possibly. pBall = p'Ball + p'Box p=mv Work Attempted Alright, What I've done has come to the conclusion I need to find normal force acting on the box which is ~ 25.5 N (25.489 N) and then found the Coefficient of friction. I know that the final velocity of the box is zero, distance is 0.57 m, the Force of kinetic friction on the box is 7 N and that the mass of the box is 2.6 Kg and that the mass of the ball is 0.142 Kg. In the end I need to find the initial velocity in m/s then convert to Mph in order to see how fast I throw. Any help as usual is greatly appreciated.