Block hits block with spring. COE and COM

[mpaandaa]

I'm doing the same problem as in this thread: https://www.physicsforums.com/showthread.php?t=70219

Homework Statement

A 3.0 kg block slides along a frictionless tabletop at 8.0 m/s toward a second block (at rest) of mass 4.5 kg. A coil spring, which obeys Hooke's law and has spring constant k = 720 N/m, is attached to the second block in such a way that it will be compressed when struck by the moving block

a. what will be the maximum compression of the spring?
b. what are the final velocities of the blocks after collision?
c. is the collision elastic or inelastic?

Homework Equations

I guessed at c and it's elastic, so both momentum and kinetic energy are conserved

COM: m_Av_0A=m_Av_FA+m_Bv_FB

COE:
1/2m_Av_0A^2=1/2m_Av_FA^2+1/2m_Bv_)B^2

Potential spring energy= 1/2kx^2

The Attempt at a Solution

a.I tried setting the spring potential energy and kinetic energy of mass A equal, but that doesn't work. I don't really know how to work with springs...
b.I tried solving the system of equations that's in the other thread, but I go something like x=0, so something is wrong there
c.it's elastic. yay =D

 

[Doc Al]

Please read the hint that I gave in the other thread. (This post: https://www.physicsforums.com/showpost.php?p=519518&postcount=6)