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Homework: Week 6 – Chapter 7 – SOLUTIONS
1.
(7.2)
Children in a tree house lift a small dog in a basket 4.70 m up to their house. If it takes 201 J of
work to do this, what is the combined mass of the dog and basket?
SOLUTION
W
=
Fd
=
mgd
( )
2
m
s
201 J
9.81
(4.70 m)
4.36 kg
W
m
gd
=
=
=
2.
(7.12) Water skiers often ride to one side of the center line of a boat, as shown in the figure above. In
this case, the ski boat is traveling at 15 m/s and the tension in the rope is 75 N. If the boat does 3500 J
of work on the skier in 50.0
m, what is the angle
θ
between the tow rope and the center line of the
boat?
SOLUTION
W
= (
T
cos
θ
)
d
cos
W
Td
=
1
1
cos
3500 J
cos
(75 N)(50.0 m)
21
W
Td
−
−
=
=
=°
3.
(7.20) At
t
= 1.0 s, a 0.40kg object is falling with a speed of v = 6.0 m/s. At
t
= 2.0 s it has a kinetic
energy of 25 J.
(a)
What is the kinetic energy of the object at
t
= 0.5 s?
(b)
What is the speed of the object at
t
= 1.0 s?
(c)
How much work was done on the object between
t
= 0.5s and
t
= 1.0 s?
SOLUTIONS
(a)
2
2
11
m
(0.40 kg) 6.0
7.2 J
22
s
Km
v
⎛⎞
==
=
⎜⎟
⎝⎠
(b)
2
1
2
K
mv
=
2
2
2
2(25 J)
11 m/s
0.40 kg
K
v
m
=
(c)
21
25 J–7.2 J
18 J
WKKK
=∆
=
−
=
=
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(7.24) A 65kg bicyclist rides his 10.0kg bicycle with a speed of 12 m/s.
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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, Mass, Work

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