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 91 kg. The mass of the rock is0.30 kg. Initially the wagon is rolling forward at a speed of 0.52m/s. Then the person throws the rock with a speed of 15 m/s. Both speeds are relative to the ground. Find the speed of the wagon after the rock is thrown(a) directly forward in one case and (b) directly backward in another.