A ball moving with a velocity hits a massive wall moving towards the ball with a velocity . An elastic impact lasts for time 

a) 
The average elastic force acting on the ball is 
b) 
The average elastic force acting on the ball is 
c) 
The kinetic energy of the ball increases by 
d) 
The kinetic energy of the ball remains the same after the collision 
A ball moving with a velocity hits a massive wall moving towards the ball with a velocity . An elastic impact lasts for time 

a) 
The average elastic force acting on the ball is 
b) 
The average elastic force acting on the ball is 
c) 
The kinetic energy of the ball increases by 
d) 
The kinetic energy of the ball remains the same after the collision 
(b,c)
In an elastic collision:
Or or
Change in momentum of ball is
Average force is
Change in KE,