While working out through some problems I encountered this problem :
A ball moving with a velocity $v$ hits a massive wall moving towards the ball with a velocity $u$. An elastic impact lasts for a time $\Delta t$
Now I have to answer whether the Kinetic energy of ball increases or remains same after collision.
In the theory books which I read, it is mentioned that Kinetic energy is conserved before and after in an elastic collision.
So that way for the above question Kinetic energy should be conserved.
But the answer given is that Kinetic energy increases.
So my question is how is it possible for Kinetic energy to increase after an elastic impact ? Is it because of the time interval $\Delta t$?