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8. In addition to the forces already shown in Fig. 621, a freebody diagram would
include an upward normal force
N
F
G
exerted by the floor on the block, a downward
mg
G
representing the gravitational pull exerted by Earth, and an assumedleftward
G
f
for the
kinetic or static friction. We choose +
x
rightwards and +
y
upwards. We apply Newton’s
second law to these axes:
0
N
Ff m
a
PF m
g
−=
+−
=
where
F
= 6.0 N and
m
= 2.5 kg is the mass of the block.
(a) In this case,
P
= 8.0 N leads to
F
N
= (2.5 kg)(9.8 m/s
2
) – 8.0 N = 16.5 N.
Using Eq. 61, this implies
,max
6.6 N
ss
N
fF
μ
==
, which is larger than the 6.0 N
rightward force – so the block (which was initially at rest) does not move. Putting
a
= 0
into the first of our equations above yields a static friction force of
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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

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