Equilibrium Problem: Solve & Find mu(min)=1

[inky]

Homework Statement

Please see attachment.

Homework Equations

Summation F(x)=0

Summation F(y)=0

Summation tau=0

The Attempt at a Solution

Please see attachment.

I know how to calculate the problem but I don't know the last part of the solution why the examinar choose mu(min)=1. Please help me.Thank you.

 

[kuruman]

You have already seen that the force of friction is the same 4N at all points. That's what's required to keep the system at equilibrium. Imagine starting with a large μ and reducing it gradually. Slipping will occur at the point where the maximum value that static friction can have (=μ R) goes just below the value 4 N first. So if you write

μR = 4 N,

at which of the three points A, B and C μR falls below 4 N first as you make μ smaller?

 

[inky]

You have already seen that the force of friction is the same 4N at all points. That's what's required to keep the system at equilibrium. Imagine starting with a large μ and reducing it gradually. Slipping will occur at the point where the maximum value that static friction can have (=μ R) goes just below the value 4 N first. So if you write

μR = 4 N,

at which of the three points A, B and C μR falls below 4 N first as you make μ smaller?

Thank you very much for explanation.Firstly I don't understand so I read many times your concept. Now I understand. I really appreciate you. Sorry for replying late.