Homework Help: Momentum question

1. The problem statement, all variables and given/known data
Two blocks are free to slide on a frictionless wooden track ABC shown in figure. The block of mass m1=5.00kg is released from A. The block of mass m2 = 10.0kg is initially at rest. The block of mass m1 collides with m2 and they start moving together.



2. Relevant equations
What will be their speed after they started moving together? How much energy is lost as a result of the collision?


3. The attempt at a solution
I know we use the conservation of momentum ( m_1*v_1 = m_1*v_f + m_2*v_2). I also know that this is an elastic equation where we can use the equation for conservation of kinetic energy.
1. The problem statement, all variables and given/known data



2. Relevant equations



3. The attempt at a solution

 

I think I figured out the speed after they started moving.
I used:
1/2*m_1*v_1^2 = m_1gh
v_1 = root(2*9.8*5.00)
v_1 = 9.90 m/s
v_1f = (m_1 -m_2) / (m_1 + m_2)
v_1f = -1/3 * 9.80
v_1f = -3.30 m/s

Is this right?

I'm not sure how to go about in solving the energy lost.