Conservation of energy or conservation of momentum

OK this is about a bullet hitting a block on wheels (inelastic collision), no friction or energy losses in the problem.

I'm trying to find Velocity of the system after collision

Why is it that if I use conservation of momentum I get a different answer of V than if I used conservation of Kinetic energy.

Momentum
m1*v1=(m1+m2)*v2

K energy
0.5*m1*v1^2=0.5*(m1+m2)*v2^2

You can make up your own numbers....I just dont understand the physics!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 PhysOrg.com science news on PhysOrg.com >> Heat-related deaths in Manhattan projected to rise>> Dire outlook despite global warming 'pause': study>> Sea level influenced tropical climate during the last ice age
 Mentor Blog Entries: 1 It's an inelastic collision--energy is not conserved. Mechanical energy will be lost.
 Recognitions: Gold Member Homework Help The reason you get different numbers is that mechanical energy is NOT conserved in an inelastic collision, while momentum is.