Newton's Three Laws (moving a box)

[southernbelle]

Homework Statement

Block A weighs 4 N and block B weighs 8N. The coefficient of sliding friction between all surfaces is 0.25. Find the force P necessary to drag block B to the left at a constant speed if
A) if A rests on B and moves with it
B) if A is held at rest
C) if A and B are connected by a light flexible cord passing around a fixed frictionless pulley.

The Attempt at a Solution

I have drawn the free body diagrams.

A) Ff=coefficient x Fn
= .25 (8 + 4)
= 3
Is that right?

B) I know the fsB = (8 + 4) coefficient of static friction
fsA = coefficient of static friction (4)

I also know that these have to be equal in order to move.
So...
fsA = fsB
But where do I put the P? On the A side or the B side?

C) I know that P must be equal to all the forces in the x-direction in order for the block to move.

The sum of the forces in the x-direction = T + fsA + fsB

so...
P = T + fsB + fsA
and
P = T + (.25)(4) + (.25)(12)
P = T + 4

How do I find T?
I know that the sum of all the forces = ma
And the acceleration in this case is 0.

But 0 = T + 4 would give T= -4.
And P =0.

I don't think that can be right.

 

[Chi Meson]

A) correct

b) I think that block A is supposed to be held in place as B is dragged out from under it.
In this case there will be two forces of friction opposing force P

C) are the two weights hanging on either side of the pulley? is one on a surface while the other is hanging? I can't "see" what this one is supposed to look like.