# Homework Help: Spring constant

1. Oct 19, 2005

### Punchlinegirl

A massless spring of length .310 m is compressed to 71.0 % of it's relaxed length, and a mass M=.150 kg is placed on top and released from rest. The mass then travels vertically and it takes 1.10 s for the mass to reach the top of its trajectory. Calculate the spring constant, in N/m. Use g= 9.81 m/s^2. Assume that the time required for the spring to reach its full extension is negligible.

I really have no idea how to do this problem. I tried using conservation of energy and solving for k, I'm not sure if I even have all of the forms of energy
(1/2)kx^2= (1/2)mv^2
(1/2)k(.2201)^2= (1/2)(.150)(10.78^2)
to get 360.0 N/m , which wasn't right

Any help would be appreciated. Thanks

2. Oct 19, 2005

### Diane_

When you multiply .310 m by 71%, what you're getting is the length of the spring when compressed. The x in the equation you're using is the distance it's compressed, not its new length.

Does that do it?

3. Oct 19, 2005

### Punchlinegirl

Yes.. that's what I was doing wrong. Thanks

4. Oct 19, 2005

### Diane_

We live to serve. ;)