# Is kinetic energy always conserved in an elastic collision/impact?

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 $$\Delta t$$?

• Does the question specifically mention kinetic energy of the *ball only after collision? Because kinetic energy of the system remains unchanged after elastic collision but the kinetic energy of the individual objects will change depending on the mass of the objects but will remain balanced. Have you looked up the wiki article on this? Aug 15, 2019 at 6:40