Slide 1

Notes: Chapter 9 Energy
Define and describe work (9.1)
Define and describe power (9.2)
Define mechanical energy. (9.3)
Define potential energy. (9.4)
Define kinetic energy and describe work-energy
theorem. (9.5)
State the law of conservation of energy. (9.6)
Describe simple machines and mechanical advantage.
Explain why no machine can have an efficiency of
100%. (9.8)
Describe the role of energy in living organisms. (9.9)
9.1 Work
• Work is done when a force acts on an object
and the object moves in the direction of the
• Work is the product of the net force working
on an object and the distance through which
the object moves.
W  Fd
• Work falls into two categories:
– To move an object against a force.
– To change the speed of an object.
• Work changes the energy state of an object.
9.2 Power
• Power is the rate at which work
is done.
• Power equals to the amount of
work done divided by the time
interval during which the work
is done.
• If a forklift is replaced with a new forklift that
has twice the power, how much greater a load
can it lift in the same amount of time? If it
lifts the same load, how much faster can it
9.3 Mechanical Energy
• Energy is the property of an object or system
that measures its ability to do work.
• Energy due to the position or movement of
something is referred to as mechanical energy.
– Two forms:
• Potential energy
• Kinetic energy
9.4 Potential Energy
• Potential energy is stored energy related to an
object’s relative position that has the ability to
do work.
• Three types of potential energy:
– Elastic potential energy
– Chemical potential energy
– Gravitational potential energy
• The amount of gravitational potential energy
possessed by an elevated object is equal to
the work done against gravity to lift it.
GPE  PEg  weight height mgh
• Height is the distance above some arbitrarily
chosen reference level.
– A position below this level gives the PEg that is
negative with respect to the reference point.
1. How much work is done on a 100-N boulder that
you carry horizontally across a 10 m room? How
much potential energy does it gain?
2. A.) How much work is done on a 100 N boulder
when you lift it 1 m?
B.) What a power is expended if you lift the
boulder a distance of 1 m in a time of 1 s?
C.) What is the GPE of the boulder in the lifted
9.5 Kinetic Energy
• A moving object has the ability to do work.
• Kinetic energy is the energy of motion.
1 2
KE  mv
• The KE of an object is equal to the work
required to bring to its speed from rest, or the
work the object can do while being brought to
1 2
Fd  mv
• When the brakes of a motorcycle traveling
60 km/h become locked, how much farther
will the motorcycle skid than if it travels at
20 km/h?
9.6 Work-Energy Theorem
• Work-energy theorem – whenever work is
done energy changes.
W  Fd
W  E
W  GPE  GPE f  GPEi  m ghf m ghi
1 2 1 2
W  KE  KE f  KEi  m vf  m vi
A car that has TWICE the KE than another car would require twice
the work to stop it. If the braking friction were the same in both
cases, how much more stopping distance would the car with twice
the KE need?
W  KE
1 2 1 2
Fd  m vf  m vi
• Kinetic energy can take several forms:
– Heat
– Light
– Sound
– 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
of energy to another but the
total energy never changes.
• When it appears that some energy is lost from
the system, it was not destroyed. The
unaccounted energy was given off as heat.
• Conservation of mechanical energy
– The total mechanical energy of a system remains
the same before during and after energy is
ME  PE  KE
MEbefore  MEafter
PEbefore  KEbefore  PEafter  KEafter
m ghbefore  m v
 m ghafter  m v
1. A 45 kg mass is dropped from a height of
10.0 m. How fast is it going when it gets to a
height of 5.0 m? How much work did gravity
have to do to get this mass up to this speed?
2. How fast would you have to jump in order to
reach a height of 0.6096 m (2 ft)?
9.8 Machines
• Machine – a device used to multiply forces or
simply change the direction of forces, or both.
– Transfers energy from one place to another or
transforms it from one form to another.
– Cannot do more work than what is put into it.
That is, it cannot create energy.
– Basic idea: Work in = Work out
• Lever – a simple
machine made of a bar
that pivots about a
fixed point called the
Win  Wout
Findin  Fout d out
• Mechanical advantage (MA) – ratio of output
force to input force for a machine. It’s the
number of time the machine multiplies force.
• Three kinds of levers:
Type I/First Cass
Type II/Second Class
Type III/Third Class
Fulcrum between effort
and load
Load between fulcrum
and effort
Effort between fulcrum
and load
• Pulley – a type of lever that is a wheel with a
groove in its rim, which can be used to change
the direction of a force exerted on a rope. A
pulley or system of pulleys can also multiply
– General rule for pulleys: the number of ropes
supporting the load equals to the mechanical
advantage (ME).
9.9 Efficiency
• Ratio of useful energy output to total energy
input. Or, also, the percentage of work input
that is converted to work output.
efficiency 
% efficiency 
– Lower the efficiency the, the greater is the energy
wasted to heat loss.
• Inclined plane – a simple machine sometimes
called a ramp.
9.10 Energy for Life
• There is more energy stored in the molecules
of food than in the reaction products (CO2 and
H2O) after the food has been metabolized.
– Process is called respiration.
• Energy enters an ecosystem when
photoautotrophs capture energy from the sun
to convert carbon dioxide and water into
– Process is called photosynthesis.
9.11 Sources of Energy
• The sun is the source of practically all our energy
here on earth.
– Fossil fuels: petroleum, coal, and natural gas
– Exceptions: nuclear energy and geothermal energy
• Solar power
– Photovoltaic (solar) cells: direct transformation to
– Wind: Indirect. Caused by uneven heating of earths
– Running water: Indirect. Caused by evaporation,
condensation, and precipitation at higher elevations
• Fuels cells: the opposite
reaction of electrolysis.
Hydrogen and oxygen are
recombined to form water.
The process can be used
to generate an electric
• Nuclear and geothermal
– Most concentrated form in
uranium and plutonium.
– Earth’s interior is kept hot
by nuclear reaction.
Basic set up for geothermal power generation.
• Power demand for an exponentially growing
human population.
– Reached 7 billion in October 2011
– Pros and cons of fossil fuels
– Pros and cons of nuclear power
– Pros and cons of wind and hydropower
– Pros and cons of solar

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