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Chapter 9 - Energy
Energy can change from one form to another
without a net loss or gain.
9.1 Work
Work is the product of the net force on
an object and the distance through which
the object is moved.
 (If/when the force causes the object to
move.)
 Work is done when a force acts on an
object and the object moves in the
direction of the force.
 Work = net force x distance
 W = Fd

9.1 Work
Work falls into two categories:
 1. Work done against a force such as
gravity, friction, or to stretch a spring.
 2. Work done to change the speed of an
object such as speeding up or slowing a
car.
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9.1 Work
Units
 Fd = W
(Force- Newton, Distance-meter,
Nm = Joule (rhymes with cool.)
 One Joule of work is done when one
Newton of force is exerted over the
distance of one meter (- like raising an
apple over your head.
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9.2 Power
Power is the rate at which (or how fast) work is
done.
 Power equals the amount of work done divided
by the time interval during which the work is
done.

Work
Power 
time
With more power, work is done faster.
 With twice the power, twice as much work can
be done in the same time, or the same amount of
work can be done in half the time.
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9.2 Power
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Units
Work
Joule
Power 

 Watt
time
second
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One Watt (W) of power is expended when
one Joule of work is done in one second.
Kilowatt=1,000 Watts,
Megawatt=1,000,000Watts
The space shuttle uses 33,000 MW of
power.
One horse-power(hp) = 765W
9.3 Mechanical Energy
Energy is the property of an object or
system that enables it to do work.
 Energy is measured in Joules (like work.)
 Mechanical Energy is the energy due to
the position or movement of something.
 The two forms of mechanical energy are
potential energy and kinetic energy.

9.4 Potential Energy
Potential Energy (PE) is energy that is
stored and held in readiness.
 Some Example include:
 Elastic Potential Energy – stretched or
compressed.
 Chemical Potential Energy – Energy
stored in the molecules of matter.
 Gravitational Potential Energy – Energy
due to elevated position.

9.4 Potential Energy
Gravitational potential energy (PEg)=
weight x height.
 PEg=mgh
 (Remember mg = weight.)
 Note that the height is the distance above
some arbitrary chosen reference level.
The gravitational potential energy is
relative to that level and depends only on
mg and h.

9.5 Kinetic Energy
Anything in motion is capable of doing
work. It has kinetic energy.
 Kinetic energy is the energy of motion.
 The kinetic energy of an object depends
on its mass and speed.
 Kinetic Energy = ½ mass x speed2

1
KE  mass X speed 2
2
9.5 Kinetic Energy
The kinetic energy of a moving object is
equal to the work required to bring it to its
speed from rest, or the work the object can
do while being brought to rest.
 net force x distance = kinetic energy.
 Fd = ½ mv2.
 Note that the speed is squared, so if the
speed is doubled, its kinetic energy is
quadrupled.
 A car moving at twice the speed has four
time the energy.

9.6 Work-Energy Theorem
The work-energy theorem describes the
relationship between work and energy.
 The work-energy theorem states that
whenever work is done, energy changes.
 Work equals the change in energy.
 WorkNet = ΔKE
 If you push a box at constant speed (with a
force equal to friction) no net work is done.
 Kinetic energy is involved in heat, sound,
light, and electricity.

9.7 Conservation of Energy
The law of conservation of energy states
that energy cannot be created or
destroyed. It can be transformed from
one form into another, but the total
amount of energy never changes.
 Our energy comes from the sun.

9.8 Machines
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A machine is a device used to multiply forces or
simply to change the direction of forces.
A machine transfers energy from one place to
another or transforms it from one form to
another.
A lever is a simple machine made of a bar that
turns about a fixed point.
Work input = work output
Since work = Fd
(force x distance)input = force x distance)output
No machine can multiply work or energy.
9.8 Machines
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Mechanical advantage of a machine is its ratio of
output force to input force.
Without friction, MA is also the ratio of input
distance to output distance.
Simple Machines:
◦
◦
◦
◦
◦
◦
Levers
Pulleys
Inclined Plane
Wedge
Screw
Wheel and Axle
9.8 Machines
Levers
 Type 1 lever – the fulcrum is between the (input) force and
the load (output force.)

◦ Ex. Seesaw
◦ Changes direction
◦ Can multiply force or distance

Type 2 lever – the load is between the fulcrum and the force.
◦ Ex. Wheelbarrow
◦ Multiplies Force

Type 3 lever – the force is between the fulcrum and the load.
◦ Ex. Hockey/Lax stick
◦ Multiplies distance (Speed?)
9.8 Machines
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Pulley (pulley system) – a kind of lever that can
change the direction of the force or multiply the
force.
Like a lever, Fulcrum, Input Force and Output
Force apply.
Mechanical advantage is equal to the number of
ropes that support the load.
Mechanical Advantage can also be determined by
the ratio of distances.
Input Dist ance
MA 
Output Dis tance
9.9 Efficiency
An Ideal Machine is 100% efficient.
In any machine, some energy is transformed into
atomic or molecular kinetic energy – making the
machine warmer.
 Efficiency of a machine is the ratio of useful energy
output to total energy input, or the percentage of the
work input that is converted to work output. (Energy
converted is work.)
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Effeciency 

Useful work output
X 100%
Total work input
The lower the efficiency of the machine, the greater
is the amount of energy wasted as heat.
9.9 Efficiency
Inclined plane is simply a sloped surface.
 MA is equal to the length divided by the
height.

length
MA 
height
Actual mechanical advantage
Effeciency 
X 100%
Theoretica l mechanical advantage
9.9 Efficiency
Complex Machines.
 A complex machine is a combination of two or more simple machines.
 Scissors – What simple machines make a pair of scissors?

◦ Two wedges and a lever.
A car jack is an example of a complex machine combining a screw and a
wheel and axle.
 An automobile engine is called a heat engine. It converts thermal energy to
mechanical energy (actually often chemical potential to thermal to
mechanical).
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Roughly 35% efficient
Energy expended through the cooling system
Exhaust gasses
Friction - Almost half heat the engine parts through friction.
Incomplete combustion
Thermal energy is the graveyard for useful energy.
9.10 Energy for Life
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Every living cell in every organism is a machine.
They, like every machine, need energy to run.
There is more energy stored in the molecules of
food than there is in the reaction products after
the food is metabolized. This energy difference
sustains life.
Cells metabolize food like engines burn fossil
fuels.
In metabolism carbon joins with oxygen to form
carbon dioxide.
9.10 Energy for Life
 The
reverse process is more difficult.
 Through photosynthesis, plants combine
carbon dioxide with water to produce
hydrocarbon compounds like sugars.
Sugar is the simplest food.
 Green plants capture the sun’s energy to
provide all other life on earth (almost.)
9.11 Sources of Energy

The sun is the source of practically all our
energy on Earth.
◦ Exceptions are nuclear and geothermal.
Fossil fuels are the byproducts of
photosynthesis hundreds of millions of years
ago.
 Solar Power – Sunlight is directly
transformed into electricity by photovoltaic
cells
 Sunlight evaporates water which falls in the
mountains.
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9.11 Sources of Energy
Uneven warming of the Earth produces
wind.
 Fuel Cells – (like a battery)
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◦ Electrolysis uses electricity to separate
hydrogen and oxygen from water H2O.
◦ Fuel cells combine hydrogen and oxygen to
make water and give off electricity.
9.11 Sources of Energy
Nuclear and Geothermal Energy.
The most concentrated form of useable energy is
stored in uranium and plutonium, which are
nuclear fuels.
 A byproduct of the radioactivity at the Earth’s
core is geothermal energy.
 “Energy sources such as nuclear, geothermal,
wind, solar, and hydro power are environmentally
friendly.”
 “The combustion of fossil fuels , on the other
hand, leads to increased atmospheric
concentrations of carbon dioxide, sulfur dioxide,
and other pollutants.”
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