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Work & Energy
Chapter 13
May 17
General Science Chapter 13
1
Work
Everyday concept of work.
Scientific definition: Work is the transfer
of energy through motion.
A force applied, in which the object
moves, to an object in the direction that
the object moves.
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General Science Chapter 13
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Work
In order for work to take place, a force
must be exerted through a distance.
In order for work to be done, there has
to be motion, and the motion has to be
in the direction of the applied force.
If there is no motion, no work will be
done
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General Science Chapter 13
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Recall “work” lab
Did you do more work lifting the books to
shoulder level or over your head?
Greater distance means more work
Did you do more work lifting 2 books or 4
books?
Greater force means more work
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Work Equation
Work  force  distance
W  F d
Work, like energy, is measured in joules.
1 J = 1 N ∙ m.
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Work and Energy
Work is the transfer of energy through
motion.
When 1 J of work is done on an object, 1 J
of energy has been transferred to the
object.
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Example
A student’s backpack weighs 10 N. She
lifts it from the floor to a shelf 1.5 m high.
How much work is done on the backpack?
Force, F = 10 N
Distance, d = 1.5 m
Work = (Force)(Distance)
Work = (10 N)(1.5 m)
Work = 15 N ∙m = 15 J
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Example #2
A dancer lifts a 400-N ballerina overhead a
distance of 1.4 m and holds her there for
several seconds. How much work is done
on the ballerina?
Work = (400 N)(1.4 m)
Work = 560 J
Note that the time was not important for us to
determine the work done.
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Example #3
A carpenter lifts a 45-kg beam 1.2 m high.
How much work is done on the beam?
Remember that weight equals mass times
acceleration due to gravity.
Weight = (45 kg)(9.8 m/s2) = 441 N
Work = (441 N)(1.2 m)
Work = 529.2 J
May 17
General Science Chapter 13
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Power



The rate at which work is done.
How much work is done in a given
amount of time
The ratio of work to time
Work
Power 
time

May 17
W
P
t
General Science Chapter 13
10
Watts




May 17
Power is measured in Watts, named
after James Watt, who helped develop
the steam engine.
1 W = 1 J/s
Very small unit, so we often use kW.
745.6 Watts = 1 horsepower
General Science Chapter 13
11
Example

A figure skater lifts his partner, who
weighs 450 N, 1 meter in 3 seconds.
How much power is required?
W F d
P

t
t
450 N 1 m
P
3s
P  150 W
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You try

A 25 000 N elevator rises 30.0 m in
60.0 s. How much power is required?
Express your answer in kW.
25000 N  30.0 m
P
60 s
P  12500 W  12.5 kW
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General Science Chapter 13
13
Making Connections
4.184 Joules = 1 calorie
Joules are units for energy and work
1 Calorie = 1000 calories
A Calorie is used for foods, so if a candy bar
has 250 Calories it is the same as 250000
calories.
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Example #1
Tommy eats a candy bar that has 230
Calories. How many Joules is that?
1 Calorie = 1000 calories
1 calorie = 4.184 Joules
so
1 Cal = 4184 Joules therefore
230 Calories = 962320 Joules
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Example #1 continued
How much Power can be produced with
the 230 Calorie candy bar in 1 hour?
230 Cal = 962320 Joules
Use the equation below
W
P
t
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Example #1 continued
962320 J
P
3600 s
May 17

P  267.3 W
P 267.3 W or 0.3585 hp
General Science Chapter 13
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Discussion #1
Define work and what are the SI units?
What units are used to measure Power?
Why is the unit kW used more often than
W?
What is the conversion factor for
horsepower to Watts?
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Machine
A device that makes work easier
By using a machine you DO NOT DO
LESS work.
It just makes it seems easier to do the
work.
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Work and machines
Work is made easier by doing 1 of 3 things
Changes the size of the input force
Changes the direction of the force
Changes both the size and the direction
of the force
Opening a paint can with a screwdriver
• Changes size – you can use less force
• Changes direction
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Forces and machines
Effort force (Fe) – applied to the machine
The force you exert
Also called input force
Resistance force (Fr) – applied by the
machine to overcome gravity or friction
The force the machine exerts
Also called output force
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Work and machines
Work input (Win) – work done on the
machine
Effort force times distance it moves
Win = Fe X de
Work output (Wout) – work done by the
machine
Resistance force times distance it moves
Wout = Fr X dr
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Mechanical advantage
The number of times a machine multiplies
the effort force
The ratio of output to input. (usually a
force)
Fr
MA 
Fe
de din
MA  or
dr dout
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Mechanical advantage
Can be greater than 1
Opening paint can
Can be equal to 1
Raising blinds
Can be less than 1
Hitting a baseball
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Example
A worker applies an effort force of
10 N to pry open a window that has a
resistance of 500 N. What is the
mechanical advantage of the crowbar?
Fr = 500 N
Fe = 10 N
Fr 500 N
MA 

Fe 10 N
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MA  50
General Science Chapter 13
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You try
A jack is used to lift a 2000-N rock. The
effort force is 200 N. Find the mechanical
advantage.
Fr 2000 N
MA 

Fe
200 N
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General Science Chapter 13
MA  10
26
Discuss #2
A _____________ is a device that makes
work easier.
What are the 3 ways a machine can make
work easier?
What do we call the force applied to a
machine?
What do we call the force applied by a
machine?
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Discussion #2
What is mechanical advantage?
What does it mean when the MA value is
equal to 1?
How do we calculate MA?
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Simple machine
A device that does work with only one
movement
There are six types
Levers
Inclined Plane
Pulley
Wedge
Wheel & Axle
Screw
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Ideal Mechanical Advantage
The ratio of output to input (usually a
force) disregarding friction and gravity.
When the output work = the input work.
Machine would be 100% efficient, which is
impossible.
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Levers
Examples
Crowbars
Seesaws
Baseball bat
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Definitions
A lever is a bar that is free to pivot, or
turn, about a fixed point.
A fulcrum is the fixed point of a lever.
The effort arm is the part of the lever on
which the force is applied.
The resistance arm is the part of the lever
that exerts the resistance.
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Lever
Effort force
Resistance force
Effort arm
Resistance arm
fulcrum
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Mechanical advantage
Review, we learned that
Fr
MA 
Fe

We can also use for levers
length of effort arm
Le
IMA 

length of resistance arm
Lr
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You try
You use a crowbar 160 cm long as a lever
to lift a large rock. The rock is 20 cm from
the fulcrum. You push down on the other
end of the crowbar.
What is the length of the effort arm?
The resistance arm?
What is the IMA of the lever?
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First class levers
The fulcrum is in the middle
Seesaw
crowbar
Effort force
Resistance arm
fulcrum
May 17
Effort arm
General Science Chapter 13
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Second class levers
The resistance is in the middle
wheelbarrow
nutcracker
Effort force
Resistance arm
May 17
fulcrum
General Science Chapter 13
Effort arm
37
Third class levers
The effort is in the middle
Baseball bat
broom
Effort force
Effort arm
Resistance arm
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General Science Chapter 13
fulcrum
38
Discuss #3
What is a lever?
What is a fulcrum?
What is the effort arm?
What is the resistance arm?
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Discussion #3
What are the 3 types of levers?
What is an example of each type of lever?
Which type usually has a IMA value < 1?
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Pulleys
Pulley – grooved wheel with a rope or
chain running along the groove
Acts like a lever
The two ends of the rope are the effort arm and
the resistance arm
The wheel acts like the fulcrum
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Fixed pulley
Attached to something that doesn’t move
Change the direction of a force
IMA of 1
Lr
Le
Fr
May 17
Fe
General Science Chapter 13
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Movable pulley
Attached to the object being moved
IMA greater than 1
Effort distance must be greater than
resistance distance
Le Fe
Lr
May 17
Fr
General Science Chapter 13
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Block and tackle
System of fixed and movable pulleys
Has IMA equal to the number of ropes that
support the resistance weight
Count every rope coming off the movable
pulleys that supports or moves the resistance
force.
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Examples of Block & Tackle
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Example of Block & Tackle #2
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Wheel and axle
Consists of two wheels of different sizes
that rotate together
The effort force is usually applied to the large
wheel
The small wheel, or axle, exerts the resistance
force
Examples: doorknob, water faucet, gears,
pencil sharpener
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Wheel and axle
Can be thought of as a lever attached to a
shaft
Radius of wheel is effort arm
Radius of axle is resistance arm
Center of axle is fulcrum
radius of wheel
rw
IMA 

radius of axle
ra
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Inclined plane
A ramp
Lifting something along an inclined plane
means you cover more distance than
lifting it straight up, but you get to use a
smaller force
effort distance
length
l
IMA 


resistance distance
height
h
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Screw
An inclined plane wrapped in a spiral
around a cylindrical post.
As you drive in a screw, the inclined plane
slides through the wood.
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Wedge
An inclined plane with one or two sloping
sides
Examples
Chisels
Knives
Axe blades
The material stays in place while the
wedge moves through it.
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Wedge
 Thickness, T
 Side, S
effort distance
Thickness T
IMA 


resistance distance
side
S
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Variations
All six kinds of simple machines are
variations of two basic machines
The lever
The inclined plane
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Compound Machine
A machine that is made up of 2 or more
simple machines.
Examples of compound machines
Fishing rod, pencil sharpener, an axe
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Discuss #4
What kind of simple machine is a ramp?
What is an inclined plane wrapped in a
spiral around a cylindrical post?
What kind of simple machine are chisels,
knives, and axes?
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Discussion #4
What type of pulleys have a MA = 1?
What is the difference between a fixed
pulley and a movable pulley?
What is a block and tackle?
What two groups can simple machines be
broken into?
What is a compound machine?
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Energy
Scientific definition: Energy is the ability to
cause change.
Ability to do work
Any sample of matter has energy if it can
produce a change in itself or in its
surroundings.
Energy comes in many forms and will be
classified as either Kinetic or Potential.
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Energy continued
Kinetic Forms
Radiant (solar), thermal, electrical, wind, sound
Potential Forms
Gravitational, mechanical, chemical, and
nuclear
Energy is measured in joules (J).
Named after a British scientist.
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Kinetic energy
Energy in the form of motion
Amount depends on the mass and velocity
of the object.
Greater mass at the same velocity OR
greater velocity with the same mass will
have greater kinetic energy
KE = ½mv2
5 Types (STEWS or SHEWS)
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Solar Energy (Radiant)
Electromagnetic energy that travels in
transverse waves.
Energy from the sun
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Thermal Energy
Heat energy, the internal energy in a
substance.
Caused by the vibration and movement of
atoms/molecules within substances.
Geothermal energy is a good example of this
type of energy.
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Electrical Energy
Energy produced by the movement of
electrons.
Lightning and electricity are good examples of
this form of energy.
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Wind Energy (Motion)
Energy produced from the movement of
objects from one place to another.
Do not need to see this movement, we just
know there is a change in position.
Wind and some forms of hydropower are good
examples of this form of energy.
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Sound Energy
Movement of energy through substances
using longitudinal or compressional
waves.
Obviously this is how we hear “things”
A compressional wave is like the
movement of an inch worm or an
accordion.
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Potential energy
Stored energy
Depends on its position/condition/height,
mass and gravity
4 Types (GECN)
PE = mgh
m = mass, g = gravity, h = height
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Gravitational Energy (Hydro)
Potential energy of an object due to height
above the earth’s surface.
The higher the object is, the more potential
energy it has.
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Elastic Potential Energy (Stored)
Energy stored in a spring or rubber band
or anything else that stretches.
The farther it is stretched, the greater its
potential energy.
Energy based on the position
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Chemical Energy
The energy stored in foods, fuels, and
batteries.
There must be a chemical reaction to get
the energy out.
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Nuclear Energy
Energy stored in the nucleus of an atom.
Fusion and Fission are two examples
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Mechanical Energy
The sum of potential and kinetic energy in
a system is called mechanical energy.
ME  PE  KE
Think about a roller coaster or bungee
jumping.

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Discuss #5
Energy Review
Define energy
What units are used to describe energy
What are the 2 main forms of energy
List 3 of the 4 subsets of stored energy
List 3 of the 5 subsets of moving energy
What is Mechanical energy?
May 17
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Discussion #5
Why is the first hill of a roller coaster ride
the highest?
Where would a roller coaster be moving
fastest?
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Discussion #5b
Why can you not travel in a circular loop
when on a roller coaster?
Is it possible for the second hill to be taller
than the first hill? Explain why?
When does a coaster have the most PE?
When does a coaster have the most KE?
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Conservation of Energy
Energy cannot be created or destroyed but
it can change from one form to another.
Example – Swing
Why does it stop?
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Conservation of energy
You can never get more work out than you
put in
Win  Wout
Fe  de  Fr  d r
If force increases, distance must
decrease.
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Internal energy
The total energy of all the particles that
make up a sample of matter.
Includes both kinetic and potential energy
of the particles.
The more mass a material has, the more
internal energy it has.
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Internal energy
Different materials have different internal
energies even when they have the same
mass and temperature.
This is because the particles in the
materials are arranged differently.
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Separate energies
Internal energy of a material depends on
the total energy of its particles.
Mechanical energy (kinetic and potential)
of the material itself has no effect on
internal energy.
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1st Law of Thermodynamics
The net change in energy equals the
energy transferred as work and heat.
U  W  Q
Q = Heat
W = Work
ΔU = Internal energy

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Efficiency
Measure of how much of the work put into
a machine is changed to useful work put
out by the machine.
Wout
efficiency 
100%
Win
Fr  d r
efficiency 
100%
Fe  d e
May 17

IMA
efficiency 
 100%
MA
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Efficiency
Can it ever be more than 100%?
How can we increase efficiency?
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example
 A worker pushes a 1500 N chair up an
inclined plane that is 4.0 m long and 1.0 m
high. The worker exerts a force of 500 N.
What is the efficiency of the inclined plane?
Fr  d r
efficiency 
100%
Fe  d e
4m
1m
1500 N 1.0 m
efficiency 
100%
500 N  4.0 m
1500
efficiency 
100%  75%
2000
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You try
Using a fixed pulley, you pull the rope
down 1.0 m with a force of 72 N. A
65-N object is raised 1.0 m. What is the
efficiency of the pulley?
65
efficiency 
100%  90%
72
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