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20. (a) In this situation, we take
G
f
s
to point uphill and to be equal to its maximum value,
in which case
f
s
, max
=
s
N
F
μ
applies, where
s
= 0.25. Applying Newton’s second law to
the block of mass
m
=
W
/
g
= 8.2 kg, in the
x
and
y
directions, produces
min 1
, max
sin
0
cos
0
s
N
Fm
g
f
m
a
g
θ
−+
=
=
−=
which (with
= 20°) leads to
()
min 1
sin
cos
8.6 N.
s
g
θμ
=
(b) Now we take
G
f
s
to point downhill and to be equal to its maximum value, in which
case
f
s
, max
=
s
F
N
applies, where
s
= 0.25. Applying Newton’s second law to the block
of mass
m
=
W
/
g
= 8.2 kg, in the
x
and
y
directions, produces
min 2
, max
sin
0
cos
0
s
N
g
f
m
a
g
=−
=
=
which (with
= 20°) leads to
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 Spring '08
 Any
 Physics, Mass

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