# Homework Help: Elastic potential energy of a spring

1. Feb 25, 2010

### jeunesse27

Hello,
I'd relly appreciate some guidance on this one:

1. The length of a spring increases by 7.0 cm from its relaxed length when a mass of 1.7 kg is hanging in equilibrium from the spring. Spring constant 2.38 N/cm.
How much elastic potential energy is stored in the spring?
A different mass is suspended and the spring length increases by 12.9 cm from its relaxed length to its new equilibrium position. What is the second mass?

2. Relevant equations
PE=.5kx^2

3. The attempt at a solution
.5(2.38)(7)^2=58.31

2. Feb 25, 2010

### dr_k

You must change the units for your PE calculation. Your current answer is $\rm N\cdot cm$, but you should write it in terms of $\rm J = N\cdot m$.

To figure out the unknown mass, draw a free body diagram, and then apply Newton's 2nd postulate for static equilibrium.

3. Feb 25, 2010

### inutard

What forces are there acting on the mass? Obviously there is the spring force (up), but is there another force on the mass thats keeping the spring stretched? As dr_k said, drawing an FBD will help you sort this weighty problem out.