# Homework Help: Conservation of energy - spring

1. Apr 4, 2014

### lukatwo

1. The problem statement, all variables and given/known data

Hello, I'm having a problem with this. So we drop the weight on this spring, and the question is: how much does the spring compress? The mass is 5kg, the height 0,4m, and the spring elasticity factor k=1700N/m.

2. Relevant equations

3. The attempt at a solution
I've tried to equalize the potential energies of when we drop the weight, and when it's all the way down. It looks like this: m(h+ΔL)=(1/2)*k*(ΔL)^2. I get a quadratic equation, and solve it. I get two answers one with +, and one -. I've tried to enter the positive one, and it's not correct. Should I enter the negative one, or is my whole attempt faulty?

2. Apr 4, 2014

### PhanthomJay

Looks like you left out g in your gravitational PE term, otherwise, looks good. Think positive.

3. Apr 4, 2014

### lukatwo

Should it be g*m*(h+∆L)=(1/2)*k*(∆L)^2, because if it's on both sides it's the same

4. Apr 4, 2014

Sure!