# Homework Help: Work and friction in a mass-spring launcher

1. Dec 12, 2004

### WillP

I have my Physics final tomorrow morning so I'm doing some last minute review and I just need someone to check if I have this sample problem done right.

A spring having a force constant of 2500N/m is placed around a vertical rod as shown (I don't have an electronic copy of the picture). A 0.9kg disc with a hole in it can slide along the rod. With the spring compressed by 4.0cm from equilibruim the disc is placed on top of the spring. When the spring is released, the disc rises to a maximum height of 14.5 cm above its starting point before falling back.
a) How much work is done by the spring in launching the disc?

b) How much work is done by gravity as the disc goes from its initial point to the highest point on its travel?

c) What is the average frictional force between the disc and the rod as the disc goes from its starting point to its highest point?

What I have done:
---

a) Wspring = -1/2Kx^2
= -1/2(2500N/m)(0.040)^2
= -2.0J

b) Wgravity = mgh
= (0.9kg)(9.8m/s^2)(0.145m)
= 1.3J

c) Wtotal = Wgravity + Wspring - Wfriction
-2.0J = 1.3J - 2.0J - Wfriction
-2.7 = Wfriction

W = F*d
-2.7J = Ffriction * (.145m)
Ffriction = 18.6N

I dunno this problem is really easy but I think I have it messed up anyway.
What do you think?