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, ms, 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 (Fs) or to tip (Ft). Which of these happens
first depends on ms, 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 hc above which the block tips before
slipping. The value of hc at which this transition occurs is
then used to produce a value of ms for these surfaces.