Chapter 6
Work and Energy
Presented by April Senger
Work
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Work is the product of a force acting over a
distance
More simply stated W = f x d
F is measured in Newtons & distance is meters
N x m is called a Joule or J
We also use kJ or MJ for objects that produce
large amounts of work
Presented by April Senger
Concept Check
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How much work is needed to lift an object that
weighs 500 N to a height of 4 meters?
500 N x 4 m = 2000 N· m or 2000 J
How much work is needed to lift it twice as
high?
500 N x 8 m = 4000 J
How much work is needed to lift 1000 N to a
height of 8 m?
1000 N x 8 m = 8000 J
Presented by April Senger
Power
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Power is the rate at which energy is changed from one
form to another
It is the rate at which work is done
P=W/t
The unit is J / s or Watt or W
Note that I will cross the tops of my Watts in problems
to make them look different than the W for work
Again, we use kW or MW for large amounts of Power
Presented by April Senger
Concept Check
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You do work when you do push ups. If you do
the same number of push ups in half the time,
how does your power output compare?
You should have 2 x the power
How many watts of power are needed when a
force of 1 N moves a book 2 m in a times of 1
second?
W = F x d = 1 N x 2 m = 2 J and P = W / t
P=2J/1s=2W
Presented by April Senger
Energy
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Energy is the ability to do work
Mechanical energy usually has motion like a bouncing
ball
A ball stops moving because its energy is converted
from one form to another
In this case, the mechanical energy is turned into a
different form of energy such as heat and the
compression of the ball’s air
Energy is measured in J because it is associated with
work
Presented by April Senger
Concept Check
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Person A pushes a block of
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ice up a ramp five times
longer than Person B lifts its
straight up to the same
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height. How much more
force does person B exert
when he lifts the ice?
5 x the force
Who does more work on the
ice?
They both do the same
because the distance up was B
the same in the end
If both jobs are done in the
same time, who expends
more power?
They both do the same. Same
work / Same time is Same
Power
Presented by April Senger
A
Potential Energy
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It is the energy that arises
because of an object’s
position = stored energy
PE is a form of mechanical
energy
PE can get complicated such
as calculating how much
energy is stored in a bungee
cord
In this class, we will only do
calculations for Gravitational
Potential Energy
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GPE = weight x height
GPE = m x g x h = mgh
We only use the height
straight up in meters and not
the path it takes to get there
On the last slide, both person
A & B resulted in the same
vertical change of height but
their paths were different
The unit for PE is Joules
Presented by April Senger
Concept Check
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How much work is done in
lifting a 200 N block of ice a
vertical distance of 2.5 m?
W = F x d = 200 N x 2.5 m
= 500 J
How much work is done in
pushing the same block of
ice up a 5 m long ramp? The
force needed is only 100 N
(advantage of inclined plane).
W = F x d = 100 N x 5 m =
500 J
What is the increase in the
block’s PE in each case?
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Remember N is m x g or
what we call weight
Weight is the force that
gravity pulls on a quantity of
matter
All forces are measured in N
PE = mgh or N x h
This is the same calculations
shown to the right
Both would result in 500 J of
Potential Energy
Presented by April Senger
Kinetic Energy
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It is the energy of motion possessed by moving objects
It is a form of mechanical energy
KE = ½ m · v2
If you multiply the mass by 10 the KE increases by 10
If you multiply the velocity by 10 the KE increases by
100
Kinetic energy often appears in forms such as heat,
sound, light and electricity and are over looked
Presented by April Senger
Concept Check
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A car travels at 30 km/h and has kinetic energy of 1
MJ. If it travels twice as fast, how much kinetic energy
will it have?
If the v is multiplied by 2 it reflects 22 or 4 times what
it was before = 4 MJ
If it travels 3 times as fast, what will its KE be?
3 squared is 9 times or 9 MJ
If it travels four times as fast, what will the KE be?
4 squared is 16 times or 16 MJ
Presented by April Senger
Conservation of Energy
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Energy cannot be created or destroyed; it may be
transformed from one form to another or transferred
from one object to another, but the total amount of
energy never changes
The transfers can become complex such as sunlight to
stored sugar in plants to coal
Another might be the sun’s energy being absorbed by
water, water to rain, rain to rivers, rivers to dams and
dams to electricity
The Earth’s energy is generally traced back to the sun
Another source is geothermal energy
Presented by April Senger
Concept Check
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Does an automobile consumed more energy when the
air conditioner is running, the radio is playing, or the
lights are on?
Yes, all require energy to work
If there is a row of windmills, what happens to the
wind’s speed before and after the power plant? Would it
be windier before or after the plant?
It would slow the wind by converting its KE into
electrical energy. The side before the mills would be
windier
Presented by April Senger
Machines
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Machines either redirect force or multiple it
If you could exclude friction, work input is
equal to work output
A common machine is the lever that consists of
an arm and a fulcrum
Archimedes once claimed that he could move
the world if he had a lever long enough and a
place to put the fulcrum
Presented by April Senger
Concept Check
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If a lever is arranged so that input distance is
twice the output distance, can we predict that
energy output will be doubled?
Energy can NEVER be changed because it is
always conserved
The resulting force would be doubled
Presented by April Senger
Pulleys
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Redirecting or
Multiplying force?
75 N
Output
75 N
Input
150 N
Output
75 N
Input
150 N
Block
75 N
Block
Presented by April Senger
Efficiency
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Efficiency is a ratio
Efficiency = Work Done = Output
Energy Used
Input
Even pulleys have a little friction and heat is
produced
A car is very inefficient and we only get out a
small fraction of the potential energy stored in
fuel
Where does the energy go?
Presented by April Senger
Metabolism
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Our cells are like tiny machines that convert
stored energy from food into kinetic energy
The human body also has poor efficiency
Why is this a good thing in humans but bad for
engines?
Presented by April Senger