# Please help, quick and simple. Potential Energy of Spring?

1. Oct 14, 2007

### jgibbon2

Block A has mass 1.00 kg, and block B has mass 3.00 kg. The blocks are forced together, compressing a spring between them; then the system is released from rest on a level, frictionless surface. The spring, which has negligible mass, is not fastened to either block and drops to the surface after it has expanded. Block B acquires a speed of 1.10 m/s.

a) What is the final speed of block A? Found this answer to be 3.30 m/s

b) How much potential energy was stored in the compressed spring?
- how do you find this answer??

2. Oct 14, 2007

### Staff: Mentor

How about conservation of energy. The blocks are released from rest where all the mechanical energy is stored in the spring.

Determine the combined kinetic energy of both blocks.

3. Oct 14, 2007

### jgibbon2

first of all thanks for helping....

okay... so i tried (1/2mv^2)a+(1/2mv^2)b=PEspring .... got 9.075 as a wrong answer....
did i mis-understand you?? i only have one more attempt at this problem (it is a mastering physics problem).

4. Oct 14, 2007

### rock.freak667

Momentum is also conserved...

Initial speed for both blocks....is 0....

and you have the masses and the final speed of one block...

5. Oct 14, 2007

### jgibbon2

isn't that how to find the final speed of block A? or am i missing something...
becuase i already have that....
i have all of the information for this problem and just need to find the potential energy that is stored in the spring....

6. Oct 15, 2008

### culikitakatoo

I have the same exact problem. I figured that the potential energy would be equal to the combined kinetic energies so I did:

.5*1*4.8 + .5*3*1.6 = 4.8 :( but it is wrong.

7. Oct 15, 2008

### culikitakatoo

ok! i got it! i was right i just forgot to square the velocities :)

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