A ball moving with a velocity v hits a massive wall moving towards the ball with a velocity u. An elastic impact lasts for time Δt. 1. The average elastic force acting on the ball is "m(u+v)" /"Δt." 2. The average elastic force acting on the ball is" 2m(u+v)" /"Δt." 3. The kinetic energy of the ball increases by 2mu(u+v). 4. The kinetic energy of the ball remains the same after the collision.
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