# Homework Help: Critical mu_s friction value homework.

1. Sep 28, 2006

### dudeman

Hi, I'm working on my homework right now and I've been attempting a problem for a while, I found a post about the same question in the archive but I don't know how to post there, so I figured it was ok to make a new thread. Hope that's ok. The origional post is at https://www.physicsforums.com/showthread.php?t=45394

EDIT: here's the actual problem

A woman attempts to push a box of books that has mass up a ramp inclined at an angle (alpha) above the horizontal. The coefficients of friction between the ramp and the box are (mu_k) and (mu_s). The force F applied by the woman is horizontal.

If (mu_s) is greater than some critical value, the woman cannot start the box moving up the ramp no matter how hard she pushes. Calculate this critical value of (mu_s).

I tried setting it up as an equilibrium with the net force in both directions, then solving for the normal force and the static friction force in that situation. Then to get mu_s(coefficent of static friction) I thought I could take the F_s/n = mu_s.

But that resulted in some huge long crazy thing with sines + cosines over more stuff, which ultimately didn't work.

Last edited: Sep 28, 2006
2. Sep 28, 2006

### dudeman

update, so I've been thinking, it says the force is horizontal, would that mean that the force is parallel ot the clope of the ramp, or stright from the left or right, meaning that I would have to seperate it into it's components. Also no change in status. I'm still stuck as before.