Bullet striking a block on a frictionless surface

1. The problem statement, all variables and given/known data
A 10g bullet with velocity 1000m/s strikes a 100g block at rest. What is their combined velocity? Can you work the problem with conservation of momentum? Energy?

2. Relevant equations

3. The attempt at a solution
For conservation of momentum, we have
10g\cdot 1000m/s + 100g\cdot 0m/s = 110gv_f\Rightarrow v_f = \frac{1000}{11}m/s.
How is this done with the conservation of energy? The KE right after impact is ##55g\cdot v_f^2 = ???##
Am I using the 0 PE level of the block? If that is the case, I have ##(m+M)gh=110g\cdot 9.8m/s^2\cdot h##. What is ##h##? I have two unknowns then.


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