What is the method for calculating velocities in a perfectly elastic collision?

[stupif]

1. a 1kg ball traverse a frictionless tube. the ball after falling through a height of 1.2m, strikes a1.5kg ball which initially at rest. find the velocities of the two balls, if the collision is perfectly elastic. V1= -0.97m/s V2= 3.88m/s



2. i found out the initial velocity of 1kg ball
mgh = 1/2mv^2
1(9.81)1.2= 1/2(1)v^2
v= 4.85m/s

after that, i use mu+mu = mv+mv
1(4.85)+ 1.5(0)= 1v +1.5v2

after that, what should i do to get the answer?
help me...please...

 

[gneill]

In a perfectly elastic collision KE is also conserved. This gives you a second equation in the two unknowns (velocities of the balls after collision).

 

[stupif]

what u mean?
but i use Newton's law of restitution to get the answer.
e=(v1-v2)/(u2-u1)

-4.85=v1-v2---------second equation
and i finally get the answer.