# Homework Help: Conservation of Energy- spring

1. Nov 17, 2008

### Maiia

1. The problem statement, all variables and given/known data
A 6kg block slides on a horizontal frictionless surface with a speed of 1.2 m/s. It is brought momentarily to rest when it hits a bumper that compresses a spring.

Heres a picture of the problem.

When I was doing this problem, I thought there were only two different energies: the Kinetic Energy and the Elastic Potential Energy. I assumed there was no Gravitational PE b/c there was no height involved. Therefore, I assumed (according to Law of Conservation of Energy) that you could set the Kinetic Energy equal to EPE.
(1/2)mv^2 + (1/2)kx^2= (1/2)mv2^2 + (1/2)kx2^2 which reduces to
(1/2)mv^2= (1/2) k(-Y2)
I am using -Y to denote the compression of the spring
if I plug in and solve
(1/2)(6kg)(1.2m/s ^2)= (1/2) (4)(Y2)
I get 1.341640786 m as the amount of compression. However, this answer is wrong. Can someone tell me what I did wrong?

2. Nov 17, 2008

### LowlyPion

Check your units. Spring constant is 4K*N/m

3. Nov 17, 2008

### Maiia

Oh wow that was stupid of me. Thanks!

4. Nov 17, 2008

### andrewm

Your understanding of the physics is correct, but perhaps you should check if your spring potential energy term with Y_2 is correct. Energy has units of Newton x meter, and the spring constant has units of Newton per meter.