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Homework: Week 7 – Chapter 8 – SOLUTIONS
1.
(81) Calculate the work done by gravity as a 2.6kg object is moved from point A to point B in the
Figure above along paths 1, 2, and 3.
SOLUTION
Wm
g
y
=
1
2
2
2
m
(2.6 kg) 9.81
(4.0 m)
s
m
0
(2.6 kg) 9.81
(1.0 m)
s
m
0
(2.6 kg) 9.81
(1.0 m)
s
51 J
W
⎛⎞
=−
⎜⎟
⎝⎠
++
2
2
m
0
(2.6 kg) 9.81
(2.0 m)
0
51 J
s
W
+=−
3
22
mm
(2.6 kg) 9.81
(1.0 m)
0
(2.6 kg) 9.81
(3.0 m)
51 J
ss
W
=+
−
=
−
d
2.
(82) Calculate the work done by friction as a 2.6kg box is slid along a floor from point A to point B
in the Figure above along paths 1, 2, and 3.
Assume that the coefficient of kinetic friction
between the box and the floor is 0.23.
SOLUTION
k
g
µ
=
for each step
1
2
2
2
2
2
m
0.23(2.6 kg)
9.81
(4.0 m)
s
m
0.2 (2.6 kg)
9.81
(4.0 m)
s
m
0.23(2.6 kg)
9.81
(1.0 m)
s
m
– 0.23(2.6 kg)
9.81
(1.0 m)
s
m
0.23(2.6 kg)
9.81
(1.0 m)
s
65 J
W
−3
−
−
2
0.23(2.6 kg) 9.81
(2.0 m)
0.23(2.6 kg) 9.81
(2.0 m)
0.23(2.6 kg) 9.81
(1.0 m)
29 J
W
−
−
2
m
s
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22
mm
0.23(2.6 kg) 9.81
(1.0 m)
0.23(2.6 kg) 9.81
(3.0 m)
0.23(2.6 kg) 9.81
(3.0 m)
ss
41 J
W
⎛⎞
=−
−
−
⎜⎟
⎝⎠
2
m
s
3.
(810) A force of 4.7 N is required to stretch a certain spring by 1.3 cm.
(a)
How far must this spring be stretched for its potential energy to be 0.020 J?
(b)
How much stretch is required for the spring potential energy to be 0.080 J?
SOLUTIONS
(a)
2
4.7 N
N
362
m
1.3
10
m
F
k
x
−
==
=
×
2
1
2
2
2(0.020 J)
0.0105 m
1.1 cm
362 N/m
Uk
x
U
x
k
=
=
=
(b)
Since
U
is proportional to
x
2
, the stretch must be doubled to quadruple the potential energy.
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This note was uploaded on 07/02/2011 for the course PHYS 201 taught by Professor Woodahl during the Fall '09 term at Indiana.
 Fall '09
 WOODAHL
 Physics, Gravity, Work

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