A bus is moving to the right at an absolute velocity U. You throw a water balloon at the bus at an absolute velocity V (greater than U) after the bus passes you. A person on the bus sees the balloon hit the bus at a speed of:

The person on the bus will see the balloon hit the bus at a velocity = V - U

Explanation:

There is relative motion between the water balloon and the person sitting in the bus.

Relative motion is the motion of a body with respect to another body (eg. an observer) from the point of reference of the second body (observer).

The person on the bus has a velocity which is the same as that of the moving bus = U

The velocity of the thrown water balloon (greater than U) = V

The relative motion between the person and the water balloon is the difference between their velocities (since they are both moving in the same direction) = V - U

Therefore, the person on the bus will see the balloon hit the bus at a velocity = V - U