# Homework Help: Inextensible cords passing over frictionless pulleys

1. Sep 25, 2011

### apprentice213

1. The problem statement, all variables and given/known data
the blocks shown in the figure are connected by flexible, inextensible cords passing over frictionless pulleys. at A, the coefficient fiction is 0.30 while at B, it is 0.40. compute the magnitude and direction of the friction forces and each block.
am i right ? x( please help .. wanna learn

2. Relevant equations

for block A:
teta(A)=tan^-1(o.30)
teta(A)=16.70
N 53.13 W
sine law:
T/sine53.13=300/sin36.87
T=400lbs

for block B:
teta(B)=tan^-1(0.40)
teta(B)=21.80
N 36.87 E
sine law:
T/sin36.87=200/sin53.13
T=150lbs

3. The attempt at a solution

2. Sep 25, 2011

### PeterO

Re: friction

I can't really see the set-up but can only advise that the usual calculation Friction = uR calculates the maximum possible friction. Perhaps in this case you don't need, and thus won't generate the maximum friction???

3. Sep 25, 2011

### apprentice213

Re: friction

sorry :) this is the figure . my instructor said we must find the tension(that red line) .. what should be the possible formula?

4. Sep 25, 2011

### apprentice213

Re: friction

then this the 1st thing i do

could it be solve by this figure ?

then followed by law of sine

Last edited: Sep 25, 2011
5. Sep 25, 2011

### Staff: Mentor

Re: friction

The force supporting each block in position comes in part by friction, and with a rope making up the difference. Friction may be acting up, or down, the slope, according to which way the block is tending to slide.

Last edited: Sep 25, 2011
6. Sep 25, 2011

### apprentice213

Re: friction

what would be the possible formula sir? because i cant really understand this topic although i tried my best to solve .. please help thnx

7. Sep 25, 2011

### apprentice213

Re: friction

anyway this topic is related to newtons law of motion

8. Sep 25, 2011

### Staff: Mentor

Re: friction

You should draw Free Body Diagrams for both blocks, showing the forces acting. Given the mass of a block and the expression for the net force you should be able to write an expression for the acceleration of each block. These expressions will contain the (as yet) unknown values of the tensions in the two ropes.

Next examine the original figure and determine the relationships between the two accelerations and the two tensions that will be enforced by the pulley setup. You should then have enough information to solve for the tensions.

9. Sep 25, 2011

Re: friction

thnx sir :)