# Homework Help: Compressed spring

1. Oct 2, 2006

### bearhug

A glider of mass 0.13 kg moves on a horizontal frictionless air track. It is permanently attached to one end of a massless horiztonal spring, which has a force constant of 9.1 N/m for both extension and compression. The other end of the spring is fixed. The glider is moved to compress the spring by 0.14 m and then released from rest. Calculate the speed of the glider at the point where it has moved 0.25 m from its starting point.
W=1/2kx^2
W=1/2(9.1)(0.11)^2
=0.055 J
Is this the right way to solve for W before solving for the speed? I wasn't sure if I needed to subtract .25-.14 or not. So far I've been getting the wrong answer and want to know where I'm going wrong.

The equation I use to find speed is W=1/2mvf^2-1/2mvi^2

2. Oct 2, 2006

### Hootenanny

Staff Emeritus
To find the kinetic energy of the glider you must find the change in potential energy of the spring;

$$W = \frac{1}{2}kx_{i}^{2} - \frac{1}{2}kx_{f}^{2}$$

3. Oct 2, 2006

### Chi Meson

Not. the .25 m is there to inform you that the glider has moved beyond the spring entirely. The full amount of PE from the spring is now the KE of the glider (ideally speaking, of course).