81. When it is about to move, we are still able to apply the equilibrium conditions, but (to obtain the critical condition) we set static friction equal to its maximum value and picture the normal force NFGas a concentrated force (upward) at the bottom corner of the cube, directly below the point Owhere Pis being applied. Thus, the line of action of NFGpasses through point Oand exerts no torque about O(of course, a similar observation applied to the pull P). Since FN= mgin this problem, we have fsmax= μmgapplied a distance haway from O. And the line of action of force of gravity (of magnitude mg), which is best pictured as a concentrated force at the center of the cube, is a distance L/2 away from O.Therefore, equilibrium of torques about Oproduces
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This note was uploaded on 05/17/2011 for the course PHY 2049 taught by Professor Any during the Spring '08 term at University of Florida.