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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
N
F
G
as a concentrated force (upward) at the bottom corner of the cube,
directly below the point
O
where
P
is being applied. Thus, the line of action of
N
F
G
passes
through point
O
and exerts no torque about
O
(of course, a similar observation applied to
the pull
P
). Since
F
N
= mg
in this problem, we have
f
s
max
=
μ
mg
applied a distance
h
away 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
O
produces
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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.
 Spring '08
 Any
 Physics, Force, Friction

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