# Homework Help: Static friction

1. Oct 14, 2009

### joemama69

1. The problem statement, all variables and given/known data

a mass m is supported as shown in the figure by ideal strings connected to a rigid wall and to tal mass 3m sitting on a horizontal surface. what is the minimum coefficient of static friction between the larger mass and the surface that permits the system to remain in equilibrium in the situation shown

2. Relevant equations

3. The attempt at a solution

Big block

Fx = u3mg - T = 0

Little block & strongs

Fx = T - mgsin60 = 0

T = us3mg - mgsin60

I dont think this is correct. How do u deal with the string intersection

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2. Oct 14, 2009

### rl.bhat

For big block Fx = μ*3mg - T*sin30 = 0
For smaller block Fy = .....?

3. Oct 15, 2009

### joemama69

Why is the tension have the sin 30 doesnt the tension come directly in the x plain

so is Fy = mg - Tcos30 = 0

4. Oct 15, 2009

### rl.bhat

Correct.
Eliminate T and find the value of μ.

5. Oct 15, 2009

### joemama69

u3mg -Tsin30 = 0
mg - t cos30 = 0 t = mg/cos30
u3mg - mgsin30(cos30) = 0
u = tan30/3

6. Oct 15, 2009

Correct.