Linear momentum conservation vs mecanical energy conservation

[jaumzaum]

A ball A (mass ma) with initial velocity v colides with a ball B (mb) initially stopped. A and B gets the same direction/velocity v', Calculate v'

By linear momentum conservation
ma.v = (ma + mb).v'
v' = mav(ma + mb)

But by mecanical energy conservation

ma.v²/2 = (ma + mb).v'²/2
v' = v (ma/(ma + mb))^(1/2), which is wrong

Why we can't use mecanical energy conservation, is there a energy dissipation?

 

[olivermsun]

A ball A (mass ma) with initial velocity v colides with a ball B (mb) initially stopped. A and B gets the same direction/velocity v', Calculate v'
...
Why we can't use mecanical energy conservation, is there a energy dissipation?

Well, for A and B to get the same direction/velocity v' after the collision, don't they have to stick?

Does that sound like an elastic collision?