Slipping and Tipping: Measuring Static Friction with a Straightedge

Following a discussion of forces, torques, and the conditions for static equilibrium, I tell my introductory mechanics class that I will show them how to measure the coefficient of static friction, μ_s, between the surfaces of a block and the front bench using nothing but a straightedge. After a few seconds of hushed anticipation, I nudge the block in Fig. 1 (a) gently with the straightedge, applying a horizontal force F that gradually increases from zero to a value that either causes the object to slip (F_s) or to tip (F_t). Which of these happens first depends on μ_s, the depth D of the block, and on h, the height above the bench surface at which F is applied. Starting at the bottom of the block, it tends to slip before tipping, but there is a critical height h_c above which the block tips before slipping. The value of h_c at which this transition occurs is then used to produce a value of μ_s for these surfaces.