A wagon is rolling forward on level ground. Friction is negligible. The person sitting in the wagon is holding a rock. The total mass of the wagon, rider, and rock is 93 kg. The mass of the rock is 0.27 kg. Initially the wagon is rolling forward at a speed of 0.49 m/s. Then the person throws the rock by a speed of 15m/s. Both speeds are relative to the ground. Discover the speed of the wagon after the rock is thrown (a) directly forward in one case and (b) directly backward in another.