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This is not necessarily a homework problem, but a sample problem of my book that I do not quite understand.

A rock climber with mass m = 55 kg rests during a "chimney climb," pressing only with her shoulders and feet against the walls of a fissure of width w = 1.0 m. Her center of mass is a horizontal distance d = 0.20 m from the wall against which her shoulders are pressed. The coefficient of static friction between her shoes and the wall is 1.1, and between her shoulders and the wall is 0.70. To rest, the climber wants to minimize her horizontal push on the walls. The minimum occurs when her feet and her shoulders are both on the verge of sliding. (a) what is the minimum horizontal push on the wall?

In the explanation of the problem, I get confused about their explanation of static friction:

"We want the climber to be on the verge of sliding at both her feet and her shoulders. That means we want the static frictional forces there to be at their maximum values. Those maximum values are Fs=U*N"

The part that muddles my mind is that they say they want her to be on the verge of sliding..so they use the "maximum" value. If the person exerted more force on both walls, wouldn't they still remain in static equilibrium? I would presume that they would start to slide after exerting a force less than U*N. It seems as though the book is implying that the person will start to slide if their force exertions exceed U*N.

Furthermore, for part B, the question asks "For that push, what must be the vertical distance h between her feet and her shoes?"

The explanation says you have to use the fact that the net torque must equal 0, and that you should choose an appropriate axis to simplify the problem..but what exactly is the arm to be rotated? I can't see you would find the motion arm of the gravity force as its extended line is straight down, as is an extension downwards of the origin...

A rock climber with mass m = 55 kg rests during a "chimney climb," pressing only with her shoulders and feet against the walls of a fissure of width w = 1.0 m. Her center of mass is a horizontal distance d = 0.20 m from the wall against which her shoulders are pressed. The coefficient of static friction between her shoes and the wall is 1.1, and between her shoulders and the wall is 0.70. To rest, the climber wants to minimize her horizontal push on the walls. The minimum occurs when her feet and her shoulders are both on the verge of sliding. (a) what is the minimum horizontal push on the wall?

In the explanation of the problem, I get confused about their explanation of static friction:

"We want the climber to be on the verge of sliding at both her feet and her shoulders. That means we want the static frictional forces there to be at their maximum values. Those maximum values are Fs=U*N"

The part that muddles my mind is that they say they want her to be on the verge of sliding..so they use the "maximum" value. If the person exerted more force on both walls, wouldn't they still remain in static equilibrium? I would presume that they would start to slide after exerting a force less than U*N. It seems as though the book is implying that the person will start to slide if their force exertions exceed U*N.

Furthermore, for part B, the question asks "For that push, what must be the vertical distance h between her feet and her shoes?"

The explanation says you have to use the fact that the net torque must equal 0, and that you should choose an appropriate axis to simplify the problem..but what exactly is the arm to be rotated? I can't see you would find the motion arm of the gravity force as its extended line is straight down, as is an extension downwards of the origin...

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