Two model train cars roll to the right. Both have masses of 1Kg. One has a velocity of 2m/s. It catches up with and couples to the other which had a velocity of 1m/s. What is the velocity of the combined mass? m1 v1i + m2 v2i = m1 v1f + m2 v2

Asolid cylinder and a cylindrical shell have the same mass, same radius, and turn on frictionless, horizontal axles. the cylindrical shell has light-weight spokes connecting the shell to the axle. a rope is wrapped around each cylinder and tied to blocks of equal masses that are held the same height above the ground. both blocks are released simultaneously. the ropes do not slip. which block hits the ground first? or is it a tie?