Static friction rotating an object

[Biker]

Homework Statement

Another question with the same idea, These questions bother me a lot. Hopefully you can help me figure why they solve it this way.
https://image.prntscr.com/image/aRnAD33ES2mML5JBwQBQvQ.png

it said in the question that U = 0.6 without saying if it is static or kinetic, However It is static because we are in a static course.
It wants P that makes the cylinder about to slip.

Homework Equations

F net = 0
net Torque = 0

The Attempt at a Solution

Now my explanation for the diagram was, That we could have rolling without slipping ( rotates about B and have maximum static friction at A) but that again makes it 4 unknowns. But he said that he wants the cylinder to be about to slip. So B must have static friction that is maximum and A must have maximum static friction because If for it to move it must overcome that friction. Is this right?

 

[rude man]

And the problem statement is ... ?

 

[TomHart]

Now my explanation for the diagram was, That we could have rolling without slipping ( rotates about B and have maximum static friction at A)

If rolling without slipping is possible, that requires that the disk lift up off the ground. So how can you have any friction at A when it is rolling up the wall?

 

[Biker]

If rolling without slipping is possible, that requires that the disk lift up off the ground. So how can you have any friction at A when it is rolling up the wall?

I didn't say it started rolling, We are only calculating the required force for it to starts to move. I didn't gain any angular acceleration.

And the problem statement is ... ?

"It wants P that makes the cylinder about to slip." Nothing really more.

 

[TomHart]

So B must have static friction that is maximum and A must have maximum static friction because If for it to move it must overcome that friction. Is this right?

Yes, both must have maximum friction in accordance with their normal forces.

 

[kuruman]

So how can you have any friction at A when it is rolling up the wall?

You can't, and the amount of force P needed to get the wheel rolling up the wall can be easily calculated. One needs that number to make sure that the wheel will slip first before rolling and not roll first before slipping. In other words, establish that P_slip < P_roll.

Edit: Disregard this post. See post #9.

 

[TomHart]

One needs that number to make sure that the wheel will slip first before rolling

Is it even possible for the wheel to roll up the wall (well, except for one very unique acceleration)? How can the wall apply a friction force to the wheel once the wheel leaves the floor? Is there a normal force from the wheel to the wall? Or am I confused again? :)

 

[Biker]

You can't, and the amount of force P needed to get the wheel rolling up the wall can be easily calculated. One needs that number to make sure that the wheel will slip first before rolling and not roll first before slipping. In other words, establish that P_slip < P_roll.

Wait so it is supposed to slip before rolling? Isn't it the other way around? It rolls then if we increase the force more it slips?

 

[kuruman]

Is it even possible for the wheel to roll up the wall (well, except for one very unique acceleration)? How can the wall apply a friction force to the wheel once the wheel leaves the floor? Is there a normal force from the wheel to the wall? Or am I confused again? :)

You are not confused. I did not pay enough attention to the equations. There is no normal force exerted by the wall on the wheel which is another of saying that the maximum force of static friction exerted by the wall on the wheel is zero.

 

[TomHart]

There is no normal force exerted by the wall on the wheel which is another of saying that the maximum force of static friction exerted by the wall on the wheel is zero.

Just to clarify one thing for @Biker, there is no normal force exerted by the wall on the wheel if the wheel leaves the ground. If the wheel remains on the ground, the friction force from the ground to the wheel will produce a normal force from the wall to the wheel.
@kuruman, would you please confirm (or correct) my statement above. Thank you.

 

[kuruman]

@TomHart is correct.