Hey, just wanna double check on these: Rain falls vertically into an open boxcar coasting freely on a horizontal frictionless track. As a result the momentum of the system consisting of the boxcar plus accumulated water increases. - I said true - since there is no friction, velocity would not decrease. However, mass increases and momentum increases. In any collision between one object initially moving with velocity vector v1 and another object initially moving with velocity vector v2, the vector (v1 + v2) must be conserved, since momentum is conserved during the collision and the total mass of the system is also conserved. I said false, since the more massive cart would influence the final direction and change the initial vector. A projectile of mass M is moving in the +x direction with speed V when it explodes into two fragments: a lighter one having mass M/4 and a heavier one having mass 3M/4. The heavier fragment moves in the +y direction with speed V. What is the speed of the lighter fragment? (Assume there are no external forces acting on the system). - I said it's V, since it has to balance out the initial momentum. Are those right?? Thanks!