Momentum/Elastic Potential Energy

AI Thread Summary
The discussion revolves around a physics problem involving two blocks on a frictionless surface and a spring. After the cord holding the blocks is burned, the block with mass 3M moves at 2.85 m/s, leading to the calculation of the velocity of block M, which is found to be -8.55 m/s. The participants discuss how to determine the system's original elastic potential energy without knowing the spring constant or the distance compressed. The solution involves applying conservation of momentum and the formula for elastic potential energy. The conversation concludes with a clarification on the relationship between mass and the period of a spring compared to a pendulum.
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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!
 
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remember, period is dependant on the mass for a spring. for a pendulum it isnt.
 
oh! thanks! i think i got it.
 
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