A combination lock has a 1.2 cm}-diameter knob that is part of the dial you turn to unlock the lock. To turn that knob, you grip it between your thumb and forefinger with a force of 0.53 N} as you twist your wrist. Suppose the coefficient of static friction between the knob and your fingers is only 0.11 because some oil accidentally got onto the knob.
What is the most torque you can exert on the knob without having it slip between your fingers?
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The tangential component of force produces the torque. Ft = µ*Fn, where Fn is the normal force. Fn = 0.53 N and µ = 0.11, so the maximum tangential force is Ft = 0.11*0.53. The maximum torque is Ft*r = 0.006*.11*.53 N-m = 3.5*10^-4 N-m