Thank you-- Problem: A 50 kg boy runs at a speed of 10 m/s and jumps onto a cart. The cart is initially at rest. If the speed of the cart with the boy on it is 2.50 m/s, what is the mass of the cart? I don't know what to do. This probelem seems very simple but I don't know where to start. These I know are the knowns: vo(cart) = 0 m/s vf(cart) = 2.50 m/s vo(boy) = 10 m/s vf(boy) = 2.5 m/s So I know velocities and I know the mass of the boy. Should I start off by finding the acceleration from F=ma, since I know a mass? But then I don't know how to find F. I know Fn = mg in the y component, but I don't know what I would use Fn next for. Am I supposed to use kinematics equations? But I don't know a, x, or time. Direction is needed, thanks!!