Physics problem, involving Hookes Law?

1. Apr 2, 2013

x86

1. The problem statement, all variables and given/known data
A 0.500 kg mass is resting to a horizontal spring constant of 45 N/m. Your lab partner pulls the spring back and releases it when you are not looking. When the spring reaches its equilibrium point (x=0) the velocity of the mass is 3.4 m/s.

Find how far your partner compressed the spring.

2. Relevant equations
Ep = 1/2 k x ^2
k = spring constant
Ek = 1/2 m v ^2

3. The attempt at a solution
Not really sure how to do this, since there is more than one unknown.

Ep1 = Ep2 + Ek2

They never stated the spring stops at x=0, and I'm willing to bet it doesn't. However, when I assume that Ep1 = Ek2 and assume it stops, I get 0.30 m (rounded) when the answer should be 0.19m

Am I doing this right if I just set Ep1 to Ek2? It doesn't make sense to do it like this, because it should be

Ep1 = Ep2 + Ek2

But then we don't have enough info to solve the equation

2. Apr 2, 2013

ehild

What is the elastic potential energy at the equilibrium point (x=0)?
Check the first sentence of your post. Something is missing.

ehild

3. Apr 2, 2013

Staff: Mentor

what you're really saying is that at x=0 the total energy of the system is just the KE because the spring is at its equilibrium position and hence there's zero PE and initially there's no KE and since energy is conserved.

PE1 + KE1 = PE2 + KE2 where KE1=0 and PE2=0 so PE1 = KE2 seems right.

I calculated the compression to be 0.358 m so I'm thinking maybe the book answer is wrong or there's some assumed friction component.