Download Kinetic energy

Work and
Energy:
Forms and
Changes
What is Work?

Remember that a force is a push or a pull. Work
requires both force and motion
– Work is force applied through a distance
– If you push against the desk and nothing moves, then
you haven't done any work
Work

There are two conditions that have to be satisfied
for work to be done on an object
– 1. The applied force must make the object move
– 2. The movement must be in the same direction as the
applied force

Work requires both force AND motion
Calculating Work
The amount of work done depends on the
amount of force exerted and the distance over
which the force is applied
 Work (N-m or Joule) = F (N) x d (m) where d
is distance moved in the direction of the force
 One newton-meter is equal to one joule so
the unit of work is a joule

Weight is a Force!

Remember that weight is a force caused by your
mass and gravity
– Fgravity = mg
– To lift something you have to exert a force to overcome
this force of gravity, so you are doing work on that object
When is Work Done?

Give a book a push and it slides along a table for a
distance of 1 m before it stops
– You did work on the book only while your hand was in
contact with it
Lift books and your arms apply a force upward to
move them, and because the force and distance
are in the same direction, your arms have done
work on the books
 Carry books while walking, and your arms are not
doing work. Why not?

Examples of Work
W = 60 N x 1 m = 60 J (N-m)
W = 20 N x 3 m = 60 J (N-m)
1m
60 N
Power

Running up stairs is harder than
walking up stairs and lifting books
quickly is harder than slowly
– Why? They both do the same amount of
work
– Running does the same work more
quickly
Power is the rate at which work is
done and energy is converted
 Power (J/sec or Watt)= Work (J)
Time (sec)
 The unit of power is Joules/sec, called
a Watt.

Check for Understanding
What’s work?
A scientist delivers a speech to an audience of
his peers
 A body builder lifts 350 pounds above his head


A mother carries her baby from room to room
A father pushes a baby in a carriage
 A woman carries a 20 kg grocery bag to her car?

Check for Understanding
What’s work?






A scientist delivers a speech to an audience of his
peers No
A body builder lifts 350 pounds above his head Yes
A mother carries her baby from room to room No
A father pushes a baby in a carriage Yes
A woman carries a 20 kg grocery bag to her car? No
THE FORCE AND THE MOVEMENT MUST
BE IN THE SAME DIRECTION TO BE WORK!
Force
Distance moved
Check for Understanding
How much work does it take to lift a 200 N
weight 2 m off the floor?
 How much work does it take to hold a 200 N
weight 2 m off the floor?
 How much work is done if you drop a 2.5 N
book 3 meters? What does the work?

Check for Understanding
How much work does it take to lift a 200 N
weight 2 m off the floor? 400 J
 How much work does it take to hold a 200 N
weight 2 m off the floor? 0 J
 How much work is done if you drop a 2.5 N
book 3 meters? 7.5 J What does the work?
Gravity!
 Eureka! Work

Check for Understanding
1. Two physics students, Ben and Bonnie, are in
the weightlifting room. Bonnie lifts the 50 kg
barbell over her head (approximately .60 m) 10
times in one minute; Ben lifts the 50 kg barbell
the same distance over his head 10 times in 10
seconds.
Which student does the most work?
Which student delivers the most power?
Explain your answers.
Check for Understanding
W = F x d but we need to find the gravitational force
(weight) of the barbells
Fg = m x g
Fg = 50kg x 9.8 N/kg = 500 N
Both use same force to lift the same barbell
Now calculate the work done by each:
W = 500N X 6m (total d)
Both use same work yet, Ben is the most powerful
since he does the same work in less time
P = W/d
Ben 500J/10sec = 50 watts
Bonnie 500J/60sec 8.3 watts
History of Work
Before engines and motors were invented, people
had to do things like lifting or pushing heavy loads
by hand
 Using an animal could help, but what they really
needed were some clever ways to either make
work easier or faster

Simple Machines
Ancient people invented simple machines that
would help them overcome resistive forces
 A simple machine is a machine that does work with
only one movement of the machine

– Some machines, such as bicycles, increase speed
– Some machines, such as an axe, change the direction of
force
– Some machines, such as a car jack, increase force
Simple Machines

Examples of simple
machines
–
–
–
–
–
–
Inclined Plane
Levers
Wheel and Axle
Wedge and Screw
Gears
Pulley
Inclined Plane
A flat, slanted surface
Eureka! Inclined Plane

Lever
Two parts:
Fulcrum
Bar
 Eureka! Levers

Wheel and Axle

Two parts:
 wheel
 bar
Wedges and Screws
Change downward
force into sideways
force
 Eureka! Screw and
Wheel

Gears

Wheels with teeth
Pulley

Two kinds:
Fixed
Moveable
Compound Machines

A compound machine
is one made up of
two or more simple
machines.
Efficiency

Efficiency - a measure of how much of the work put
into a machine is changed into useful output work
– Every machine is less than 100% effective
– Not 100% of the work done is useful work, because some
gets turned into other forms, like heat
– Machines can be made more efficient by reducing friction
with a lubricant, such as oil or grease, which is added to
surfaces that rub together
Mechanical Advantage

Two forces are involved when a machine is used to
do work
– One force is applied to the machine and that is the input
force
– The force applied by the machine is called the output force

Mechanical advantage of a machine is the ratio of the
output force to the input force
Mechanical Advantage

Window blinds are a
machine that
changes
force
– A downward pull on
the cord is changed
to an upward force on
the blinds
– The input and output
forces are equal, so
the MA is 1
– Eureka! Mechanical
Advantage
Energy

Energy is all around you:
– Light
– Heat
– Wind

You use energy when you:
– hit a softball
– lift your book bag
– digest food

Every change that occurs—
large or small—involves
energy
Changes Require Energy

When something is able to change its environment
or itself, it has energy
– Anything that causes change must have energy
– You use energy to arrange your hair to look the way you
want it to
– You also use energy when you walk down the halls of your
school between classes or eat your lunch
Nature of Energy

What is energy that it can be involved in so many
different activities?
– Energy- the ability to do work
– If an object or organism does work the object or
organism uses energy
– Whenever you do work you transfer energy from
one thing to another
Nature of Energy
Because of the direct connection between
energy and work, energy is measured in the
same unit as work: joules (J)
 In addition to using energy to do work, objects
gain energy because work is being done on
them

Forms of Energy

The five main forms of energy are:
– Thermal (heat)
– Chemical
– Electromagnetic
– Nuclear
– Mechanical

If you have $100, you could store it in a variety of
forms—cash in your wallet, a bank account, or coins
 Regardless of its form, money is money, and the
same goes for energy in that these are only different
forms of the same thing
States of Energy
The most common energy conversion is the
conversion between potential and kinetic energy
 All forms of energy can be in either of two
states:
– Potential
– Kinetic

Kinetic Energy
Kinetic energy- the energy of motion
 Depends on both mass and velocity

– The faster an object moves, the more kinetic energy
it has
– The greater the mass of a moving object, the more
kinetic energy it has
Ek = mass x velocity2
2
What has a greater effect on kinetic energy, mass
or velocity? Why?
Potential Energy
Even motionless objects have energy
 Potential energy- stored energy due
to interactions between objects

– If the apple stays in the tree, the
energy will remain stored
– If the apple falls, that stored energy is
converted to kinetic energy
Elastic Potential Energy

If you stretch a rubber band and let it go, it sails
across the room
– As it flies through the air, it has kinetic energy due to its
motion but where did this kinetic energy come from?
– The stretched rubber band had energy stored as elastic
potential energy

Elastic potential energy is energy stored by
something that can stretch or compress, such as a
rubber band or spring
Chemical Potential Energy

Gasoline, food, and other substances have
chemical potential energy
– Energy stored due to chemical bonds is chemical
potential energy
– Energy is stored due to the bonds that hold the atoms
together and is released when the gas is burned
Gravitational Potential Energy

Any system that has
objects that are attracted
to each other through
gravity has gravitational
potential energy
– Gravitational potential
energy (GPE) - energy due
to gravitational forces
between objects
– Water and Earth
– Apple and Earth
Gravitational Potential Energy
Depends on mass and height
 Ep = m (kg) x g (N/kg) x h (m)
where g is the force caused by gravity (9.8 N/kg)

– If you stand on a 3-meter diving board, you have 3
times the G.P.E, than you had on a 1-meter diving
board
– A person with 3 times a larger mass has 3 times the
potential energy
Gravitational Potential Energy

A waterfall, a suspension bridge, and a falling
snowflake all have gravitational potential
energy
Potential
Energy
Kinetic
Energy
Potential
Energy
Kinetic
Energy
Check for Understanding

Ek = 1 mv2
2
– What is the kinetic energy of a 100 kg man moving 5
m/s?
– What is the kinetic energy of 0.5 kg ball moving at 30
m/s?
Check for Understanding

Ek = 1 mv2
2
– What is the kinetic energy of a 100 kg man moving 5
m/s?
1 mv2 = 1 x 100kg x (5m/s)2 = 1250 J
2
2
– What is the kinetic energy of 0.5 kg ball moving at 30
m/s?
1 mv2 = 1 x 0.5kg x (30m/s)2 = 225 J
2
2

Eureka! Kinetic Energy
Check for Understanding

Ep = m x g x h
– A 100 kg boulder is on the edge of the cliff 10 m off
the ground. How much energy does it have?
– A 0.5 kg ball is thrown 15 m into the air How much
potential energy does it have at its highest point?
Check for Understanding

E=mxgxh
– A 100 kg boulder is on the edge of the cliff 10 m off
the ground. How much energy does it have?
100kg x 9.8 m/s2 x 10m = ~ 10,000 J
– A 0.5 kg ball is thrown 15 m into the air How much
potential energy does it have at its highest point?
0.5 kg x 9.8 m/s2 x 15m = ~ 75 J

Eureka! Potential Energy
The Law of Conservation of Energy

The Law of Conservation of Energy- energy can
be neither created nor destroyed by ordinary
means, it can only be converted from one form
to another


Energy can change from one form to another, but the
total amount of energy never changes
The total energy of a system remains constant
Energy Transformations

The law of conservation of energy is a universal
principle that describes what happens to energy
as it is transferred from one object to another or
as it is transformed
– You are likely to think of energy as race cars roar
past or as your body uses energy from food to help it
move, or as the Sun warms your skin on a summer
day
– These situations involve energy changing from one
form to another form
Mechanical Energy
Transformations

Mechanical energy is the sum of the kinetic
energy and potential energy of the objects in a
system
– The mechanical energy of a system remains constant
or nearly constant
– In these cases, energy is only converted between
different forms of mechanical energy
Mechanical Energy
Transformations

An apple-Earth system has gravitational
potential energy due to the gravitational force
between apple and Earth
– The instant the apple comes loose from
the tree:
 It accelerates due to gravity
 It loses height so the gravitational
potential energy decreases
 Its potential energy is transformed into
kinetic as the speed of the apple increases
 The potential energy that the apple lost is
gained back as kinetic energy so the total amount
of energy remains the same
Energy Transformation in
Projectile Motion
Energy transformations also occur during
projectile motion when an object moves in a
curved path
 However, the mechanical energy of the ball-Earth
system remains constant as it rises and falls

Energy Transformation in a
Basketball
Ball speeds up
Ball slows down
Force
of
gravity
Energy Transformation in a
Roller Coaster
At the point of maximum potential energy, the car
has minimum kinetic energy.
Total energy is conserved and constant
Energy Transformations in a
Swing

When you ride on a swing part of the fun is the
feeling of almost falling as you drop from the highest
point to the lowest point of the swing’s path
– The ride starts with a push that gets you moving, giving
you kinetic energy
– As the swing rises, you lose speed but gain height
– In energy terms, kinetic energy changes to gravitational
potential energy

The Effect of Friction
You know that if you don’t continue to pump a swing
or get a push, your arcs will become lower and you
eventually will stop swinging
 In other words, the mechanical (kinetic and
potential) energy of the swing decreases, as if the
energy were being destroyed or lost Is this a
violation of the law of conservation of energy?

The Effect of Friction

NO!!! Energy in the system is conserved
– With every movement, the swing’s chains rub on their
hooks and air pushes on the rider
– Friction and air resistance cause some of the mechanical
energy of the swing to change to thermal energy
– With every pass of the swing, the temperature of the
hooks and the air increases a little, so the mechanical
energy of the swing is
not destroyed, but
transformed into
thermal energy, or heat
Conservation of Energy with a
Pendulum
When is the pendulum moving the fastest?
at the lowest point
Conservation of Energy with a
Pendulum
When does the pendulum have the most kinetic energy?
at the lowest point
Conservation of Energy with a
Pendulum
When does the pendulum have the most
gravitational potential energy?
at the highest point
Conservation of Energy with a
Pendulum
P
K
E
Conservation of Energy with a
Pendulum
P
K
E
Gravitational potential energy depends on height
Conservation of Energy with a
Pendulum
P
K
Kinetic energy depends on speed
E
E=P+K
P
K
E
Total Energy Doesn’t Change
P
+
K
=
E
A Pendulum
All KE
PE
No KE
PE
No KE
Transforming Electrical Energy
Light bulbs transform electrical energy into light
so you can see
 The warmth you feel around the bulb is
evidence that some of that electrical energy is
transformed into thermal energy, or heat

Transforming Chemical Energy

Fuel stores chemical potential energy
– An engine transforms chemical
potential energy of gasoline
molecules into the kinetic energy
of a moving car or bus
– Several energy conversions occur in this
process
– In a car, a spark plug fires, initiating the
conversion of chemical potential energy
into thermal energy
– As the hot gases expand, thermal energy is
converted into kinetic energy
Transforming Chemical Energy

Some chemical energy
transformations are less obvious
because they do not result in
visible motion, sound, heat or
light
– Every green plant you see converts
the radiant energy from the Sun
into the energy stored in chemical
bonds in the plant, like in
carbohydrates (sugars)
– When we eat we transform the
potential energy stored in the
carbohydrate bonds and transform
it into kinetic energy
Other Energy Conversions
In a battery, chemical energy is converted into
electromagnetic energy
 The mechanical energy of a waterfall is converted to
electrical energy in a generator

Chemical  Thermal Mechanical
Check for Understanding
A
E
B
C
D
Where is the gravitational potential energy maximum?
Where is the kinetic energy maximum?
Where is the gravitational potential energy minimum?
Where is the kinetic energy minimum?
Check for Understanding
A
E
B
Where
and E
Where
Where
Where
C
D
is the gravitational potential energy maximum? A
is the kinetic energy maximum? C
is the gravitational potential energy minimum? C
is the kinetic energy minimum? A and E