Physics
Chapter 10
Work, Energy and Simple
Machines
Chapter 10
Work, Energy and Simple
Machines
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10.1 Energy and Work
10.2 Machines
10.1 Energy and Work
Energy
 The ability for an
object to produce a
change in itself or in
its surroundings
 Units-Joules
10.1 Energy and Work
Energy
Many forms of energy:
 Thermal
 Chemical
 Electrical
 Mechanical
10.1 Energy and Work
Energy
Two types of
Mechanical Energy
 Kinetic energy
(KE)
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Energy of motion
Potential energy
(PE)

Stored energy
(more on this in Chapter 11)
10.1 Energy and Work
Work
 The process of
changing the energy
of a system
 The transfer of
energy by
mechanical means
 Units-Joules
(energy)
10.1 Energy and Work
Work
To calculate work:
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W=Fxd
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Where:
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W = work (J = Nm)
F = force (N)
d = distance (m)
also W = mad
also W = mgd
10.1 Energy and Work
Work
Example:
 If a weight lifter applies
150 N of force to a
barbell to raise it 0.8m,
how much work does
she do?
Answer:
 120 J
10.1 Energy and Work
Work-Energy Theorem
 Doing work on an object
will increase or decrease
its energy
 Work causes a change in
energy that is equal to
the work done
 This is the WorkEnergy Theorem

W = E
10.1 Energy and Work
Work
 Since work is the
transfer of energy by
mechanical means, there
is a direct relationship
between work and
energy as long as force
applied to do the work is
in the same direction as
the motion of the object.
10.1 Energy and Work
Work
 When the force that
does the work is applied
at an angle, only the
portion of the work that
is in the direction of
motion is used to
calculate the work done.
10.1 Energy and Work
Work
 When the person pushes
the lawn mower only the
horizontal component
(Fh) of the force applied
(Fa) is used to calculate
the work done.


So…. W = F cos d
Unless the lawn mower Fv
is lifted!!!
Fa
Fh
10.1 Energy and Work
Work
Example:
 A police dog applies
215N of force on his
leash at a 35° angle to
the ground. If he pulls
his handler 28m, how
much work does he do?
Answer
 4931 J
10.1 Energy and Work
Work
Under which set of
circumstances is work
done?
 Picking up an apple
 Carrying an apple
 Putting down an apple
 Dropping an apple
 Eating an apple
Who does the work?
10.1 Energy and Work

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What kind of work
does friction do?
Is there such a thing
as negative work?
10.1 Energy and Work
Power
 The rate of doing
work
 The rate at which
energy is transferred
 Watt is the unit for
power (W = J/s)
10.1 Energy and Work
Power
Can be calculated by
the equation:

P = W/t
Or

P = Fd/t
Or

P = mad/t =mgd/t
10.1 Energy and Work
Example:
 If a motor applies 1500
N of force to a
rollercoaster car to lift it
up a 22m hill in 0.60
minutes, what is the
power it produces?
Answer:
 917 W
10.1 Energy and Work
Homework:
P.242; 19-28
10.2 Machines
Machine
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A device that makes work “easier”
It aids in the transfer of energy from one
place to another
It is not a source of energy
It will not increase the work done/energy
transferred
It only changes the direction and/or
magnitude of the force applied to the
machine
10.2 Machines
Machine
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The work that is done on the machine (work
you do) is called the input work (Win)
Input work is equal to effort force (Fe)
times effort distance (de)
Win = Fe x de
Effort force is force put into the machine
Effort distance is distance you move by
using the machine
10.2 Machines
Machine
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The work that is by the machine (work
machine does) is called the output work
(Wout)
Output work is equal to resistance force
(Fr) times resistance distance (dr)
Wout = Fr x dr
Resistance force is force put out by the
machine
Resistance distance is distance the
machine moves the object
10.2 Machines
Simple Machine
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A machine is in its simplest form
There are two general types of simple
machines:
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Inclined Plane
Lever
10.2 Machines
Inclined Plane
 A simple machine
made up of a
sloping plank
10.2 Machines
Inclined Plane
Win = Fe x de
dr
Wout = Fr x dr
Fr = wt = mg
(object)
10.2 Machines
Inclined Plane
 A simple machine that makes work
easier by increasing the force out of the
machine by increasing the distance the
force must be applied into the machine
 So Fe < Fr and de > dr
 It can also change the direction of the
force
10.2 Machines
Inclined Plane
 Other types of inclined planes
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
Wedge
Screw
10.2 Machines
Lever
 A simple machine made up of “lever
arms” that rotate around a fulcrum
10.2 Machines
Lever
Win = Fe x de
Wout = Fr x dr
10.2 Machines
Lever
 A simple machine made up of “lever
arms” that rotate around a fulcrum
 Can either increase or decrease the
effort force by decreasing or increasing
the effort distance
 Also it can change the direction of the
forces
10.2 Machines
Lever
 Other types of levers
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
Pulley
Wheel and Axle
10.2 Machines
How machines work
 A machine just transfers energy
Ideal machine—no energy is lost (?), all
the energy put into the machine comes
out of the machine
 So since the energy is transferred 100%
by the machine Win = Wout
10.2 Machines
How machines work (ideal machine)
Win = Wout
 Fe x de = Fr x dr
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So if the machine increases the effort
force (Fe < Fr) it must decrease the
effort distance (de > dr) to keep work
input equal to work output
10.2 Machines
How machines work (ideal machine)
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Fe x de = Fr x dr can be rewritten as:
Fr/Fe = de/dr
Ideal mechanical advantage: shows how a
machine changes the motion (distance
moved) of the forces applied to it.
--it’s the property of a machine (how its
designed)
—IMA
= de/dr
10.2 Machines
How machines work (real machine)

Does Fe x de =
machine? Huh?
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Fr/Fe ≠ de/dr
Fr x dr for an real
Mechanical advantage—how much a
machine changes the force applied to it
--you must use machine to determine MA!
Mechanical advantage—MA
= Fr/Fe
10.2 Machines
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But in real machines some energy is
always lost by the machine
So Win > Wout always
Because of this we can judge how
“good” a machine is by determining its
efficiency
10.2 Machines
Efficiency—the percentage of energy that is
transferred out of a machine
Efficiency = Wout/Win (x 100%)
Or
Efficiency = MA/IMA (x 100%)
10.2 Machines
Lets try some practice problems:
How much force is needed to push a
45 N box up an inclined plane that
is 18m long (diagonal) and 8 m
high? (assume it is frictionless—
ideal machine)
10.2 Machines
How much force is needed to push a 45 N
box up an inclined plane that is 18m
long (diagonal) and 8 m high? (assume
it is frictionless—ideal machine)
Answer: 20 N
What is the machine’s ideal
mechanical advantage?
10.2 Machines
How much force is needed to push a 45 N box
up an inclined plane that is 18m long
(diagonal) and 8 m high? (assume it is
frictionless—ideal machine)
Answer: 20 N
What is the machine’s ideal mechanical
advantage?
Answer: 2.25
This is also its mechanical advantage
(why?)
10.2 Machines
Lets try some practice problems:
30 N of force is needed to push a 45
N box up an inclined plane that is
18m long (diagonal) and 8 m high.
What is the machine’s ideal
mechanical advantage?
10.2 Machines
30 N of force is needed to push a 45 N
box up an inclined plane that is 18m
long (diagonal) and 8 m high.
What is the machine’s ideal
mechanical advantage?
Answer: 2.25
What is its mechanical advantage?
10.2 Machines
30 N of force is needed to push a 45 N
box up an inclined plane that is 18m
long (diagonal) and 8 m high.
What is the machine’s ideal mechanical
advantage?
Answer: 2.25
What is its mechanical advantage?
Answer: 1.5
What is its efficiency?
10.2 Machines
30 N of force is needed to push a 45 N box up
an inclined plane that is 18m long (diagonal)
and 8 m high.
What is the machine’s ideal mechanical
advantage?
Answer: 2.25
What is its mechanical advantage?
Answer: 1.5
What is its efficiency?
Answer: 66.7%
10.2 Machines
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So remember a machine makes work
seem easier not by increasing the
amount of work that can be done (or
energy) but by increasing the
magnitude and/or direction of the force
put into the machine (how?)
10.2 Machines
Lets try some practice problems:
P. 210; 13, 14, 15
Homework
P. 242; 29-31, 48-52