I know how to do this question but just I cannot understand about the question asking 4kg ball. The question is " A 2 kg ball moving at 7m/s strikes a 1 kg ball at rest.The 2kg ball continues to move in the same direction at 3m/s.

a) Find the velocity of the 4kg ball.

b) How much the kinetic energy lost.

c) Find the coefficient of restitution.

I know how to do b) but a) and c) completely I don't know.Can someone show me the steps/guidelines to start with.And from the guidelines I will do myself I know.Thanx

I can understand your confusion! The question makes no mention of a 4kg ball! Since it gave you the velocity of the 2kg ball after, I can only guess that it meant to ask you about the 1kg ball. The '4' must have been a mistake.

Once you calculate final velocity for the 1kg ball, you can do part c with the following formula:

C_R=\frac{v_{1kg.final}-v_{2kg.final}}{v_{2kg.initial}-v_{1kg.initial}}

The question probably had a mistake in it, in that the 4 kg was supposed to be 1 kg, like this:

A 2 kg ball moving at 7 m/s strikes a 1 kg ball at rest. The 2 kg ball continues to move in the same direction at 3 m/s.
a) Find the velocity of the 1 kg ball.
b) How much the kinetic energy lost.
c) Find the coefficient of restitution.

Can you give (a) a try now?

EDIT: That'll teach me open a window and start doing installations and pixel artwork before answering

Thank you very much jcaron2

Thank you very much Unknown008