Kinetic Energy and Colliding Blocks

[mohabitar]

A 2.4 kg block with a kinetic energy of 48 J slides on a horizontal frictionless table and collides with a second block. The second block is made of a different material and is initially at rest. The two blocks stick together and after the collision have a kinetic energy of 36 J. Find the mass of the second block.

What I did was first find the speed of the first block:
1/2MV^2=48
V[1,i]=5.98
Then the sum of their kinetic energies at the end is just 36. I use that sum to isolate for M[2], and I get .38, which is the incorrect answer. The correct answer is .80 kg. Any idea what I'm doing wrong or what I'm supposed to do? I'm practicing for an exam and this question was one of the old test questions.

Thanks

 

[mohabitar]

Man what happened to this forum..this thing used to actually be helpful

 

[collinsmark]

Kinetic energy is not conserved (some of the energy is lost to heat when the blocks stick together). You need to use conservation of momentum.

(You can use KE = (1/2)mv² as a way of finding the velocities, but the rest of the problem is conservation of momentum, not energy.)