Homework Help: Newton's Laws, Forces

1. Oct 2, 2007

skateza

1. The problem statement, all variables and given/known data
A block with a mass of 3.1kg is placed at rest on a surface inclined at an angle of 45 degrees above the horizontal. Th coefficient of static friction between the block and the surface is 0.50, and a force of magnitude F pushes upward on the block, parallel to the inclined surface.
a) The block will remain at rest only if F is greater than a minimum value Fmin, and less than a maximum value, Fmax. Explain the reasons for this behaviour
b) Calculate Fmin
c) Calculate Fmax

3. The attempt at a solution
Well i found a) to be IF you don't apply enough force (Fmin) the weight of the box will pull it down the surface, IF you apply too much force (Fmax) you will overcome the static friction of the box, pushing the box up the ramp.

My problems are in parts b and c;

I know that Fs is whatever it has to be in order to keep the object at rest until it reaches Fsmax. I also know once it is at max it is equal to UsN (0.5*mgcos45). So part c was really not to hard, just solve Fmax = mgsin45 - UsN, i got 10.75N

but in part b), if the box is not falling down the ramp than F >or= Fs, so i set Fs equal to F and solved the x component of the vector, 2F = mgsin45, i got 10.75N... How can i have too values that are identical for Fs and Fsmax... where did i go wrong?

2. Oct 2, 2007

Kurdt

Staff Emeritus
So the force to overcome static friction you have calculated is $F_s=0.5\times mgcos(45)$. The component of weight acting parallel to the surface is $mgsin(45)$. So the minimum force required will be the force due to the weight acting parallel to the surface minus the force needed to overcome static friction.

Now do you think you can go on from there and say what the maximum force will be?