Elastic potential energy of a spring

[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?

Homework Equations

PE=.5kx^2

The Attempt at a Solution

.5(2.38)(7)^2=58.31

 

[dr_k]

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?

Homework Equations

PE=.5kx^2

The Attempt at a Solution

.5(2.38)(7)^2=58.31

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.

 

[inutard]

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