119.
CONCEPT QUESTIONS
a.
The block is in equilibrium in each case.
Since the block moves at a constant
velocity in each case, it is not accelerating.
A zero acceleration is the hallmark of
equilibrium.
b.
The direction of the kinetic frictional
force is not the same in each case.
The
frictional force always opposes the
relative motion between the surface of
the block and the wall.
Therefore, when
the block slides upward, the frictional
force points downward.
When the block
slides downward, the frictional force
points upward.
These directions are
shown in the freebody diagrams (not to
scale) for the two cases.
In these diagrams
W
is the weight of the block and
f
k
is the
kinetic frictional force.
c.
In each case the magnitude of the frictional force is the same.
It is given by
Equation 4.8 as
f
k
=
μ
k
F
N
, where
μ
k
is the coefficient of kinetic friction and
F
N
is the
magnitude of the normal force.
The coefficient of kinetic friction does not depend
on the direction of the motion.
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 Spring '08
 SOFFER
 Force, Friction

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