# Homework Help: [Friction] Angled bar pressing down against a plate

1. Nov 1, 2015

### bartersnarter

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

This is my problem in all its glory.

2. Relevant equations
f = uN

3. The attempt at a solution
For part a I simply did that tan(alpha) = f/N and therefore alpha = arctan(f/N).
This simplifies to alpha = arctan(u). I believe this part is correct.

The second part of this question is what gets me. I set up the sum of the moments for the bar.
My equation is: W*sin(alpha)*L/2 - N*sin(alpha)*L + f*cos(alpha)*L = 0

Algebra gave me that N = - W*sin(alpha) / [2*(u*cos(alpha) - sin(alpha)]

I made a FBD for the plate and the equations I got were that F - f = 0 and Ngroundonplate - N = 0. (The plain N being the normal force of the bar on the plate)

Since f = uN, substitution of N gave me f = - u*W*sin(alpha) / [2*(u*cos(alpha) - sin(alpha)] and then dividing top and bottom by cos(alpha) I got that f = INFINITY.

If this makes sense, can anyone explain how? If it's wrong can anyone point me in the right direction?

2. Nov 1, 2015

### haruspex

Which way is f acting on the bar?

3. Nov 1, 2015

### bartersnarter

If the workpiece is moving to the left, the friction force should be towards the right, shouldn't it?

4. Nov 1, 2015

### haruspex

Is that the friction force on the workpiece or the friction force on the bar?

5. Nov 1, 2015

### bartersnarter

The friction on the workpiece.

6. Nov 1, 2015

### haruspex

Ok, but the equation I quoted is in regard to forces on the bar, no?

7. Nov 1, 2015

### bartersnarter

Ahh, I see now! I had set up that sum of moments for part a and I neglected to alter it for part b. Thanks a bunch!
After altering it I get F = uW/4. This makes much more sense!