Ph 111
NAME ____RWB
October 10, 2008
Quiz 3 Solutions
Please use the value of 9.8 m/s
2
for the acceleration of
gravity, and give your numerical answers to at least three
significant figures.
A crate with a mass of
m
= 10 kg is sliding at a constant
speed
v
1
along a frictionless horizontal table, when it
encounters a friction patch, with coefficient of kinetic
friction
μ
k
= 0.4 .
After the crate has slid for a distance
d
= 8 m, its speed is
v
2
= 11 m/s.
Histogram, Quiz 3
0
10
20
30
B
i
n01234567891
0
Bin
Frequency
Ph 111.01, Fal l 2008.
110 peopl e; aver age = 5.3/ 10
C or better
(a)
Find the work done by the normal force over the distance
d
.
The normal force is perpendicular to the displacement  so there is no work done.
0.00
J
(b)
Find the work done by friction over the distance
d
.
The frictional force is
( )( )( )
2
0.4 10 kg
9.8 m/s
39.2 N
fN
m
g
μμ
==
=
=
The work done is negative, given by
313.6
J
()
(
)
N 8 m
313.6 J
=
−
nc

 39.2
Wf
d
(c)
Find the speed
v
1
of the crate when it first encounters the friction patch.
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 Fall '08
 Staff
 Acceleration, Force, Gravity, Mass, 9.8 m, 9.8 m/s

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