# Homework Help: Hints please! friction, normal force

1. Aug 4, 2011

### jehan4141

Hints please! :) friction, normal force

Please, can someone give me hints on how to look at this problem? I'd really like to uncover this problem myself, so please do not give away the answer to me! :D

You can see the drawing here, on problem 66:
http://img25.imageshack.us/img25/8769/pict0466.gif [Broken]
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A block is pressed against a vertical wall by a force P. This force can either push the block upward at a constant velocity or allow it to slide downward at a constant velocity. The magnitude of the force is different in the two cases, while the directional angle is the same. Kinetic friction exists between the block and the wall, and the coefficient of kinetic friction is 0.250. The weight of the block is 39.0 N, and the directional angle for the force P = 30.0. Determine the magnitude of P when the block slides a) up the wall and b) down the wall.

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First off, is the "directional angle" the angle between the block and the P vector? Or is it the angle between the P vector and the imaginary horiztonal axis? It is placed so ambiguously in the drawing.

Ok so, I understand that the force acting upon the block are weight, friction, force P, and the normal force (Fn) provided by the wall.

I do not understand how the P can differ in case A (up the wall) and case B (down the wall).

The problem states that the box will slide up and down the wall at a constant velocity. But if P is different in both cases, there will be acceleration, no?

I can figure the force of P when the block P remains motionless. I can do this by setting all the horizontal forces equal to zero and all of the vertical forces equal to zero. But I do not understand how the block can move up or down the wall without acceleration.

Thank you in advance for any help, and sorry this was so incredibly long! :)

Last edited by a moderator: May 5, 2017
2. Aug 4, 2011

### Staff: Mentor

Re: Hints please! :) friction, normal force

Good. Which way do they act in each case?

P is whatever it needs to be to produce a constant velocity. Hint: What other forces change when the block is moving up versus moving down?

You are assuming that all other forces are the same. Not so. (Consider the direction of the forces.)

Zero acceleration just means no change in velocity. Once it's moving up or down at a constant speed, the acceleration is zero.

3. Aug 4, 2011

### jehan4141

Re: Hints please! :) friction, normal force

Ah thank you sososo much!!! I see that the frictional force vectors have different directions for each case. :D THANK YOU you are wonderful. It's these little, obvious things that trip me up in physics.

4. Aug 4, 2011

### jehan4141

Re: Hints please! :) friction, normal force

Also, do you think that the "directional angle" is the angle between the P force vector and the horizontal? Or the angle between the block and force vector for P? I have no answer to check my work and that would give me a totally different answer.

5. Aug 4, 2011

### Staff: Mentor

Re: Hints please! :) friction, normal force

Looks to me that it's the angle between the block and the force vector, just going by where they put the theta in the diagram. (But it is ambiguous.)

6. Aug 4, 2011

### jehan4141

Re: Hints please! :) friction, normal force

:) thank you