CHAPTER 5: WORK AND
MACHINES
WORK


WORK IN THE
SENSE OF SCIENCE
IS DIFFERENT THAN
WHAT MOST
PEOPLE CONSIDER
WORK AS BEING.
Work: the transfer of
energy when a force
makes an object
move.
TWO CONDITIONS FOR WORK
1)
2)
The applied force
must make the
object move.
The movement
must be in the
same direction as
the applied force.
For Example
When lifting books off the ground, you do
work on the books because books move
upward and the force is also upward.
 If you hold the books in your arms and do
not move, then no work is being done.
 If you start to move, you will not be doing
any work on the books because you are
moving horizontally, but the force of your
arms is still upward.

ENERGY and WORK
When work is done, a transfer of energy
always occurs.
 In other words, ENERGY IS THE ABILITY
TO DO WORK.
 An object can transfer energy to another
object by doing work on that object.
 Energy is always transferred from the
object that is doing the work to the object
on which the work is done.

WORK and POWER
Work is calculated as:
 Work (J) = Force X distance or
 W = Fd

Power is the rate at which energy is
transferred or the amount of work done
per second and is measured in W or
watts.
 P=Work / time or P =E/t

TWO SYSTEMS OF WORK
The English System
 Force F= pounds
 Distance d= feet
 Time
t = seconds
 Work W = Fd = footpounds = ftlb
 Power P = W/t = ftlb/s
 Horsepower 1HP = 550 ftlb/s

METRIC SYSTEM
Force F = Newton = N = 1kgm/s²
 Distance
d = meter = m
 Time t = second
 WORK W = Fd = Newtonmeter = Nm = J
 POWER P= W/s = NM/s = J/s = Watt =W
 Horsepower 1 HP = 746W

WORK – POWER – HORSEPOWER DIAGRAM
WORK – POWER – HORSEPOWER DIAGRAM
WORK
W=Fd
WORK – POWER – HORSEPOWER DIAGRAM
WORK
W=Fd
POWER
P=W/t
WORK – POWER – HORSEPOWER DIAGRAM
WORK
W=Fd
M
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S
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POWER
P=W/t
WORK – POWER – HORSEPOWER DIAGRAM
WORK
W=Fd
M
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S
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T
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POWER
P=W/t
E
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WORK – POWER – HORSEPOWER DIAGRAM
F=Newton =N
d=meter
F=pound
WORK
W=Fd
M
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T
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S
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S
T
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POWER
P=W/t
d=foot
E
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WORK – POWER – HORSEPOWER DIAGRAM
F=Newton =N
d=meter
F=pound
WORK
W=Fd
TIME
M
E
T
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S
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S
T
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POWER
P=W/t
d=foot
E
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WORK – POWER – HORSEPOWER DIAGRAM
F=Newton =N
d=meter
F=pound
WORK
W=Fd
Work = Nm = J
Newtonmeter = Joule
TIME
M
E
T
R
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S
Y
S
T
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POWER
P=W/t
d=foot
E
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S
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WORK – POWER – HORSEPOWER DIAGRAM
F=Newton =N
d=meter
F=pound
d=foot
WORK
W=Fd
Work = Nm = J
Newtonmeter = Joule
Work = footpound
ftlb
TIME
M
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T
R
I
C
S
Y
S
T
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M
POWER
P=W/t
E
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S
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S
T
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WORK – POWER – HORSEPOWER DIAGRAM
F=Newton =N
d=meter
F=pound
d=foot
WORK
W=Fd
Work = Nm = J
Newtonmeter = Joule
Work = footpound
ftlb
TIME
M
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T
R
I
C
S
Y
S
T
E
M
P = Nm/s = J/s = Watt
POWER
P=W/t
E
N
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S
Y
S
T
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M
WORK – POWER – HORSEPOWER DIAGRAM
F=Newton =N
d=meter
F=pound
d=foot
WORK
W=Fd
Work = Nm = J
Newtonmeter = Joule
Work = footpound
ftlb
TIME
M
E
T
R
I
C
S
Y
S
T
E
M
P = ftlb/s
P = Nm/s = J/s = Watt
POWER
P=W/t
E
N
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L
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S
H
S
Y
S
T
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M
WORK – POWER – HORSEPOWER DIAGRAM
F=Newton =N
d=meter
F=pound
d=foot
WORK
W=Fd
Work = Nm = J
Newtonmeter = Joule
Work = footpound
ftlb
TIME
M
E
T
R
I
C
S
Y
S
T
E
M
P = ftlb/s
P = Nm/s = J/s = Watt
POWER
P=W/t
746 watts = 1 Horse Power = 550 ftlb/s
E
N
G
L
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S
H
S
Y
S
T
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Example from page 128

You push a refrigerator with a force of
100N. If you move the refrigerator a
distance of 5 m, how much work did you
do?
Section 2: Using Machines
Machine: a device that makes
doing work easier.
Work is Made Easier . . .




by increasing the force applied.
 A hammer used as a lever increases force.
by changing the distance over which the force is
applied.
 An inclined plane increases distance but
decreases force applied to do the same work.
by changing the direction of an applied force.
 A pulley changes the direction of the pull on
the rope.
Applied Force = is the force put into the
machine
Work Input = Effort Force
A force is any push or pull.
 You are doing work when you use a force
to cause motion.
 Machines make work easier, but they
need energy to do work.
 A person is usually the source of energy
for a simple machine.
 The force applied to the machine is called
work input or effort force = Win

Work Output = Resistance Force
Machines do work, too.
 The machine exerts a force over a
distance.
 This is called output force.
 The work a machine does is called work
output = Wout
 The work output is used to move a “force”
you and the machine wish to move or
overcome.
 This is the resistance force or the load.

Machines Work through a Distance
The distance that is applied to a machine
such as a pulley rope or an inclined
plane’s length is called the effort distance.
 The distance that the machine moves the
object is called the resistance distance.
 Both of these are important when
calculating such things as work,
mechanical advantage, and efficiency.

Recap
Work input = work done by the user of the
machine
 Effort Force = the force done by the user of
the machine
 Effort Distance = distance put into machine
 Work output = work done by machine
 Resistance Force = he weight of the object
to be moved by the simple machine
 Resistance Distance = distance the object is
moved

Conserving Energy
Energy is always conserved.
 Wout is never greater than Win.
 A machine does not transfer all of the
energy it receives to the object.
 Some of the energy is transferred to
heat through friction.
 So, Win is always greater than Wout.

Ideal Machines
 Win
= Fin x din
 Wout = Fout x dout
 For an ideal machine, meaning no loss
to friction, Win = Wout
Or
 Fin x din = Fout x dout
 See page 135
Page 135
A
hammer claw moves a distance of
1cm to remove a nail. If the output
force of 1,500 N is exerted by the
claw, and you move the handle of the
hammer 5 cm, find the input force.
Mechanical Advantage
The ratio of the output force to the input
force
 MA = Fout /Fin
 Ideal mechanical advantage is MA without
friction
 IMA = din/dout

Efficiency
Efficiency = the measure of how much of
the work put into a machine is changed
into useful output work
 Efficiency (%) = output work / input work X
100%
 Ideal machine = 100%
 Real machine = less than 100%
 Lubricant reduced friction and increases
efficiency.
