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.
A
The average elastic force acting on the ball is [m(u+v)]Δt.
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B
The average elastic force acting on the ball is [2m(u+v)]Δt.
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C
The kinetic energy of the ball increases by 2mu (u + v)
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D
The kinetic energy of the ball remains the same after the collision.
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Solution
The correct options are B The average elastic force acting on the ball is [2m(u+v)]Δt. C The kinetic energy of the ball increases by 2mu (u + v) In an elastic collision : vsep=vapp or v′−u=v+u or v′=v+2u Change in momentum of ball is |Pf−Pi|=|m(−v′)−mv|=m(v′+v)=2mu(u+v) Average force is ΔPΔt=2m(u+v)Δt Change in KE, Kf−Ki=12mv′2−12mv2=2m u (u + v)