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## 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.