We have a block of height b and base width
a resting on a horizontal surface. And a gradually
increasing horizontal force is applied to
the block. The condition for sliding is F>mu*m*g
where mu is the coefficient of friction between
the block and the ground. m is the mass of
the block and g is the acceleration due to
gravity. To find the toppling condition we
considered torques about the bottom left edge
of the block. Whether the block slides or
topples first depends on which condition is
satisfied first. If the block sometimes slides
and sometimes topples then both conditions
must be near equality. If both conditions
are at equality then we just have a pair of simultaneous equations 
to solve to get the coefficient of friction.
