# Homework Help: A Spring Problem

1. Nov 2, 2008

### lebprince

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

A horizontal spring with stiffness 0.8 N/m has a relaxed length of 17 cm (0.17 m). A mass of 21 grams (0.021 kg) is attached and you stretch the spring to a total length of 25 cm (0.25 m). The mass is then released from rest. What is the speed of the mass at the moment when the spring returns to its relaxed length of 17 cm (0.17 m)?

2. Relevant equations

Ef= Ei +W

Kf + Uf = Ki + Ui + W

3. The attempt at a solution

so no work done W=0
Ki = 0 since released from rest
then 1/2 mvf^2 + 1/2Ksf^2 = 1\2Ksi^2
i want to find vf , i have m , i have k, i guess am having a problem with Sf and Si cause am not getting a correct answer, and am not sure if am using the right formula. Thanks

2. Nov 2, 2008

### kmikias

ok what is the formula when you have a spring .....late me give you hint.

Kf + Uf = Ki + Ui
you don't have to find work because there is no friction .so Uf and Ui is Zero because the hight is 0.so what left

Kf = ki
which is 1/2mv^2 = 1/2kx^2

plug the value you have k,x,and also mass
don't forget there is some trick on X.
good luck .ask if you have question

3. Nov 2, 2008