Solving Henry Sincosky's Static Friction Problem

[seraphimhouse]

Homework Statement

In about 1915, Henry Sincosky of Philadelphia suspended himself from a rafter by gripping the rafter with the thumb of each hand on one side and the fingers on the other side (Fig. 6-22). Sincosky's mass was 82.0 kg. If the coefficient of static friction between hand and rafter was 0.700, what was the least magnitude of the normal force on the rafter from each thumb or opposite fingers?

http://edugen.wiley.com/edugen/courses/crs1650/art/qb/qu/c06/pict_6_22.gif

Homework Equations

Fnet = Ma
Fs,max = UsFn

Us = Coefficient of static friction

The Attempt at a Solution

With Normal force and Gravitational force pointing down and Static Friction pointing up on the free-body diagram, I for Fs - Fn - Fg = 0 [Zero acceleration]. Then I put Fg = Fs - Fn and substituted Fs with UsFn. Got the answer 2680 [rounded to 3 sig figs] but was the incorrect answer.

I have a feeling that it's my free body diagram that's wrong.

 

[tascja]

i think that for the fingers of the hand, your normal force would be pointing up from the board and your frictional force to the side.. because friction always opposes motion, and if the fingers were to slide off the plank they would move sideways.. that's if i understand the situation correctly...

 

[seraphimhouse]

Maybe this picture will help:

http://edugen.wiley.com/edugen/courses/crs1650/art/qb/qu/c06/pict_6_22.gif

 

[tascja]

oh ok.. you i was thinking about it wrong... so for this pic i think that the frictional force points up, the normal force to the side and gravity down...

 

[seraphimhouse]

I was wondering, why normal force to the side?

 

[tascja]

well normal force is always perpendicular to the surface of contact, so from the board to his fingers...