Momentum/Elastic Potential Energy

[hellomister]

Homework Statement

Two blocks of masses M and 3M are placed on a horizontal, frictionless surface. A light spring is attached to one of them, and the blocks are pushed together with the spring between them as shown in the figure below. A cord initially holding the blocks together is burned; after that happens, the block of mass 3M moves to the right with a speed of 2.85 m/s.

Image: http://www.webassign.net/serpse/serpse7_p9-4.gifa. What is the velocity of block mass M? Assume Right is positive and left is negative
I got the solution to this. It was -8.55 m/s
b. Find the system's original elastic potential energy, taking M= .300 Kg
This one i don't understand how you can find the elastic potential energy without knowing the spring constant/distance compressed.

Homework Equations

Pinitial=Pfinal
P=MV

Us= 1/2 KX^2

The Attempt at a Solution

I got the solution for part a.
m1v1+m2v2=0
v1=-m2/m1 *v2
v1=-3m/m*(2.85)
v1=-8.55

part b.

potential energy for a spring is 1/2KX^2
The problem gives the mass M as .300kg therefore mass 3M= .900kg
1/2kx^2=1/2MV^2+1/2(3M)V^2

I took a chance and i solved it and put it in and i was right!

thank you physicznoob a lot!

 

[physicznoob]

remember, period is dependent on the mass for a spring. for a pendulum it isnt.

 

[hellomister]

oh! thanks! i think i got it.