# Homework Help: Force of a spring and static friction

1. Sep 28, 2011

### matt@USA

1. The problem statement, all variables and given/known data
A backpack full of books weighing 54.3N rests on a table in a physics laboratory classroom. A spring with a force constant of 200N/m is attached to the backpack and pulled horizontally, as indicated in the figure. If the spring is pulled until it stretches 1.50cm and the pack remains at rest, what is the force of friction exerted on the backpack by the table? Imagine the spring being pulled with a force to the -x, and the force of static friction going in the +x direction.

2. Relevant equations
I know that the force of the spring is equal to F=kx, the force of static friction is mus*Normal force. I tried to find the mus to solve the equation but I was wrong. I got the sum of the net force in the x direction as fs-F=0, and the sum of the net force in the y direction as N-mg=0.

3. The attempt at a solution
The final equation I used to try to solve for mus was mus=kx/N, with 200N/m being my k, .015m being my x, and 54.3N being my N. Can someone please tell me where, or how I messed up?

2. Sep 28, 2011

### PeterO

How far off was your answer? 200 N/m is a pretty weak spring. 200 kN/m would be a good solid spring.

3. Sep 28, 2011

### matt@USA

NM, I figured it out. The correct equation was fs=F, and F=kx, so fs=kx!