Physics
posted by Lilly .
Two blocks, each with a mass = 0.168 kg, can slide without friction on a horizontal surface. Initially, block 1 is in motion with a speed = 1.28 m/s; block 2 is at rest. When block 1 collides with block 2, a spring bumper on block 1 is compressed. Maximum compression of the spring occurs when the two blocks move with the same speed, /2 = 0.640 m/s. If the maximum compression of the spring is 1.55 cm, what is its force constant?

At maximum compression, the spring potential energy will equal the drop in Total kinetic energy. Total KE goes from (M/2)Vo^2 to 2*(M/2)*(Vo/2)^2
= (M/4)Vo^2/4
M is the mass of one block; Vo is its initial speed of block 1
(M/2)Vo^2/4 = (k/2)X^2
Solve for k 
Im still confused. Ive tried solving for k a couple of different ways and the website tells me that I am wrong. My answers have ranged from .0286 to 791.1 N/m. Ive tried converting 1.55 cm to m, and it still says that I am wrong.

X = 1.55*10^2 m
M = 0.168 kg
Vo = 1.28 m/s
k = (Vo/X)^2*M = 1146 N/m
You apparently did not use the formula I provided.