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Chapter 9
Energy
The Big Idea
Energy can change from
one form to another
without a net loss or gain.
Discussion
What is energy?
 Pros and cons?
 How many different forms of energy can
you think of?
 Take 1 minute with an elbow partner and
write down as many things you know
about energy

What exactly is it???



Persons, places, and things have energy, but
we observe only the effects of energy when
something is happening.
Only when energy is being transferred from one
place to another or transformed from one form
to another.
Energy = the property of an object or a system
that enables it to do work.
Work is related to energy!

work = net force X distance (W = Fd)

When you apply a force on an object and
it moves, you’ve done work.

Falls into 2 categories:
1. work done against another force (gravity/friction)
2. work done to change the speed of an
object (slow it down/speed it up)
Units of Work

Remember, Work = Force x Distance.

What are the units for work?
Newton x meters

1 N•m = 1 Joule (J)
(1 J of work is done when 1 N of F is exerted
over a distance of 1 m; also kJ and MJ are
common units)
Power
Power equals the amount of work done
divided by the time interval during which
the work is done.
 Tells you how fast the work is done!

Units for Power

Remember
Work is measured in Joules
 Power is measured in
Joules per Second

James Watt – 18th c developer of steam engine
One watt (W) of power is expended when one
J of work is done in one sec.
One kW = 1000 W; 1 MW = 1,000,000 W.
One horsepower (hp) = 0.75 kW.
QUESTION:
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 operate?
P = W/t
 2 P = 2 W/t
 2 P = W/(.5 t)

Mechanical Energy
The energy due to the position of
something or the movement of
something.
 The two forms of mechanical energy are
kinetic energy and potential energy.

Potential Energy (PE)

Energy that is stored and held in readiness;
it has the potential to do work.
 Elastic Potential Energy
• A stretched or compressed spring
• A stretched rubber band
• A bow in a bow and arrow

Chemical Energy
• Is released when a chemical change takes place.
• Fossil fuels, electrical batteries, food we eat

Gravitational Potential Energy
• Energy of raised position in a gravitational field.
• Water in an elevated reservoir, raised ram of a
pile driver
Gravitational Potential Energy
Amount of gravitational PE possessed by
an elevated object is equal to the work
done against gravity in lifting it.
 W = Fupwarddvertical
 F = weight = mg; so…
 Gravitational PE = weight X height
Or

PE = mgh
Kinetic Energy (KE)
Energy of motion
 Depends on mass and speed of object


Kinetic energy = 1/2 mass X velocity squared
Or
Kinetic Energy
The KE of a moving object is equal to the
work required to bring it to that speed
from rest, or the work the object can do
while being brought to rest.
 Net force x distance = KE OR Fd = ½ mv2

*note that speed is squared, so if the speed of an
object doubles, its KE is quadrupled!
*Consequently, it takes 4x the work to double the
speed and 4x as much work to stop
Applications of KE
Accident investigators are aware that a
car going 100km/h has 4x the KE it would
have at 50km/h. Therefore, it will skid 4x
as far when its brakes are locked.
 Speed limits/braking distances are
determined by accounting for this, along
with driver’s reaction times.
 When the brakes of a motorcycle traveling
at 60km/h lock, how much farther will the
bike skid than if it travels at 20km/h?

Total Mechanical Energy

the total mechanical energy accounts for
both PE and KE of an object


Add the kinetic and potential energy
ME = KE + PE
Law of Conservation of Energy
“Energy cannot be created or destroyed.
It can be transformed from one form into
another, but the total amount of energy
never changes.”
 The sum of PE and KE is the same
anywhere
along the
path of the
object.

Work-Energy Theorem
Work changes KE and PE
 If no change in energy occurs, then no
work is done
 Work = ΔE

Try it.
An object has a mass of 10kg.
 It is 5m off the ground.
 It is moving 2m/sec.

Find its gravitational potential energy.
 Find its kinetic energy
 Find its total mechanical energy

Answers:
m = 10kg; h = 5m; speed = 2m/s given
Solve for:
 gPE = mgh = (10kg)(9.8m/s2)(5m) = 490J


KE = ½ mv2 = ½ (10kg)(22) = 20J

ME = KE + PE = 20 + 490 = 510J
Machines
A device use to multiply forces or change
the direction of forces
 Built on the idea of conservation of energy
 Neglecting heat from friction…
Work INPUT = Work OUTPUT
 Transfers energy from one place to
another or transforms it from one form to
another

Simple Machines
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.
 The fulcrum is the fixed point that a lever
operates on.

Mechanical Advantage.
work input = work output
 The ratio of output force
to input force for a machine
is called the
mechanical advantage.

Try it!

Find the mechanical advantage.
Mechanical Advantage
The answer is 8! (80N)/(10N)
 It can also be determined by the ratio of
input distance to output distance.
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3 basic types of levers:
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Class 1 – F is applied in
opposite direction of how
load moves
ex. See saw
Class 2 – F and load
move in same direction
ex. Wheelbarrow
Class 3 – F and load
move in same direction
ex. Elbow joint
Pulleys
A kind of lever that can be used to change
the direction of a force
 Used in systems, they can change the
direction and multiple the force.

Rule of thumb:
Mechanical advantage of a simple pulley
system (same size pulleys on same shaft)
is equal to the number of strands of rope
that actually support the load.
 Law of conservation of energy still applies
to very complex pulley systems;
Mechanical advantage can also be found
from the ratio of distances:
(input distance)/(output distance)

Efficiency
Anytime Work is done energy changes.
 No machine can put out more energy than
is put into it.
 No machine can create energy, only
transfer/transform it!
 “Ideal” are 100% efficient (All work input is
transferred to work output.)
 Not realistic!

Efficiency
In any machine, some energy is
transformed into atomic/molecular KE –
warms up the machine.
 This “wasted” energy is thermal energy;
lost as heat.
 The lower the efficiency of a machine, the
greater the amount of energy wasted as
heat.

Efficiency
Efficiency = useful work output
total work input
OR
Efficiency = actual mechanical advantage
theoretical mechanical adv.
*actual m.a. accounts for friction, theoretical m.a.
disregards friction
*efficiency is expressed as a % so remember to multiply
decimal by 100.
Even the best engines are unlikely
to be more than 35% efficient!
<15% of the PE from the gas is converted
in KE of the moving car
 Much of the
PE is converted
to thermal
energy and is
lost
