Download Solutions Statics Simple Machines Ch 6

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Solutions Statics Simple Machines Ch 6
Questions:
1. Some types of physical labor, particularly if it involves lifting objects, such as shoveling dirt or carrying
shingles up to a roof, are “work” in the physics sense of the word. Or, pushing a lawn mower would be
work corresponding to the physics definition. When we use the word “work” for employment, such as “go
to work” or “school work”, there is often no sense of physical labor or of moving something through a
distance by a force.
4.
The woman does work by moving the water with her hands and feet, because she must exert a force to
move the water some distance. As she stops swimming and begins to float in the current, the current does
work on her because she gains kinetic energy. Once she is floating the same speed as the water, her kinetic
energy does not change, and so no net work is being done on her.
6.
While it is true that no work is being done on the wall by you, there is work being done inside your arm
muscles. Exerting a force via a muscle causes small continual motions in your muscles, which is work, and
which causes you to tire. An example of this is holding a heavy load at arm’s length. While at first you
may hold the load steady, after a time your arm will begin to shake, which indicates the motion of muscles
in your arm.
Problems:
1.
The force and the displacement are both downwards, so the angle between them is 0o.

WG  mgd cos    265 kg  9.80 m s 2
3.
  2.80 m  cos 0
 7.27 103 J
o
x
(a) See the free-body diagram for the crate as it is being pulled. Since the
crate is not accelerating horizontally, FP  Ffr  230 N . The work done to
move it across the floor is the work done by the pulling force. The angle
between the pulling force and the direction of motion is 0o.
Ffr
WP  FP d cos 0o   230 N  4.0 m 1  9.2  10 2 J
FP
mg
FN
(b) See the free-body diagram for the crate as it is being lifted. Since the crate is not
accelerating vertically, the pulling force is the same magnitude as the weight. The
angle between the pulling force and the direction of motion is 0o.
y
WP  FP d cos 0o  mgd  1300 N  4.0 m   5.2  103 J
7.
mg
Consider the diagram shown. If we assume that the man pushes
straight down on the end of the lever, then the work done by the
(the “input” work) is given by WI  FI hI . The object moves a
FI
shorter distance, as seen from the diagram, and so WO  FO hO .
Equate the two amounts of work.
FO hI
FO hO  FI hI 

FI hO
But by similar triangles, we see that
748961322
hI
hO

lI
lO
, and so
FO
FI
6/18/2017 10:04:00 AM

hI
man
lI
lO
lI
lO
FP
hO
FO
.
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