Imagine you are on a cart that rests on a frictionless surface (you + car = 100 kg) with 30 basketballs (1 kg each). You throw the basketballs off with a speed of 3 m/s relative to the cart every ten seconds. After 5 minutes (when all the basketballs have been thrown), how fast in m/s are you going? You throw the basketballs in the same direction each time.