Say a car is moving with speed u. The guy in the car chucks a ball with speed v relative to himself. The gain in KE,as recorded by him will be mv2/2. For an observer on the ground, i.e. non-moving frame, the change in kinetic energy is [(v+u)2 - u2]m/2 = m[v2 + 2uv]/2. Why is there a difference in energy gained? I am not able to put my finger on it.