# Homework Help: What is the Force Constant k of a Spring?

1. Oct 29, 2013

### ImmaGrump

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

A 4.50 ✕ 10^5 kg subway train is brought to a stop from a speed of 0.500 m/s in 0.900 m by a large spring bumper at the end of its track. What is the force constant k of the spring?

M- 4.50E5 kg
IV- .5m/s
FV- 0m/s

2. Relevant equations
k=mg/x
F=-kx
PE = (.5)kx^2

3. The attempt at a solution
k=(450000kg)(9.81m/s^2)/ .9 = 4900000, which isn't the right answer. Can someone help me out?

Last edited: Oct 29, 2013
2. Oct 29, 2013

### Princu

Have you read anything about the concept of potential energy of a spring?This question is directly based on that..
The kinetic energy of the train is being converted in the potential energy of spring?

3. Oct 29, 2013

### ImmaGrump

Well I know PE = (.5)kx^2, but Im afraid I don't know how to plug in the numbers.

4. Oct 29, 2013

### Hollumber

What's the intial kinetic energy (KE) of the train?
KE will = 0 when v = 0 (why is this true?)
Due to conservation of potential energy, initial(KE+PE) = final(KE+ E)