# Homework Help: Test Question (Potential Energy): Need help finding answer

1. Apr 18, 2005

### gokugreene

I can't figure out what I did wrong on this test question.

A 4kg block is dropped from a height of 0.8m above a spring, k=120N/m. Find the maximum compression of the spring.

I have the PE at the top is equal to KE at the bottom.
PEtop=31.4J
KEbottom=31.4J

The potential energy in the spring when compressed should equal the KE at the bottom. Based upon that I should be able to find the maximum distance the spring will compress.

KEbottom=Wspring
31.4J=.5*120*x^2
Rearranging x=sqrt(2*31.4/120) and I get x=-0.723m

What am I doing wrong here? Can someone point it out.

Thanks, peace

2. Apr 18, 2005

### SpaceTiger

Staff Emeritus
Remember that the spring is compressing in the vertical direction, so there is more gravitational potential energy to take into account:

Initial gravitational potential energy = Final spring potential energy

$$mg(x_0+x_{spring})=\frac{1}{2}kx_{spring}^2$$

Last edited: Apr 18, 2005
3. Apr 18, 2005