Download Energy and work

ENERGY
AND WORK
What is energy?
Discuss
A REMINDER!
Energy cannot be made or destroyed, only transformed
(changed).
Energy is measured in Joules.
That’s me!
James Prescott Joule
1818-89
WHAT IS ENERGY?
“The capacity to do work”
Not a very good
definition!
WORK
In physics,
work has a special
meaning, different to
“normal” English.
WORK
In physics,
work is the
amount of energy
transformed
(changed) when a
force moves (in
the direction of
the force)
WORK
For example, if Mr J pushes a table, he is doing work
against the friction force of the table against the floor.
CALCULATING WORK
The amount of work done (measured in Joules) is
equal to the force used (Newtons) multiplied by the distance
the force has moved (metres).
Force (N)
Distance travelled (m)
WORK DONE = FORCE X DISTANCE
Another physics formula to learn!
Copy it down!!
IMPORTANT!
The force has to be in
the direction of movement.
Carrying your shopping
home from (insert
supermarket name here) is
not work in physics!
Not
working
REMEMBER!
There are three key ingredients to work –
force
displacement
cause
In order for a force to qualify as having
done work on an object, there must be a
displacement and the force must cause the
displacement.
GROUP ACTIVITY:
Why am I
doing this?
READ THE STATEMENTS BELOW
WHICH ONES ARE EXAMPLES OF WORK BEING DONE?
1. Mr J applies a force to a wall and becomes exhausted.
2. A book falls off a table and free falls to the ground.
3. A waiter carries a tray full of meals above his head
straight across the room at constant speed.
4. A rocket accelerates through space.
LIFTING OBJECTS
When we lift objects,
we are doing work because a
force is moving.
Force
Distance
moved
LIFTING OBJECTS
Our lifting force is equal to the weight of the object.
Lifting force
weight
Let’s look at
some examples
GROUP CHALLENGE
WORK DONE (J) = FORCE (N) X DISTANCE (M)
A woman pushes a car with a force of 400 N for a distance of
15m. How much work has she done?
WORK DONE (J) = FORCE (N) X DISTANCE (M)
A woman pushes a car with a force of 400 N for a distance of
15m. How much work has she done?
Work = force x distance = 400 x 15 = 6000 J
GROUP CHALLENGE
WORK DONE (J) = FORCE (N) X DISTANCE (M)
A man lifts a mass of 120 kg to a height of 2.5m. How much
work did he do?
WORK DONE (J) = FORCE (N) X DISTANCE
(M)
A man lifts a mass of 120 kg to a height of 2.5m. How much
work did he do?
Force = weight = 1200N
Work = F x d = 1200 x 2.5
Work = 3000 J
WORK AGAINST
GRAVITY.
As energy can not be created or destroyed, when you do
work against gravity it must be transferred.
If you lift an object you increase its gravitational potential
energy (g.p.e).
The total amount of
g.p.e gained must be
equal to the amount of
energy transferred, or
work done against
gravity.
Lets derive
So work done:
W = Fxd
What would the Force on the sack of
potatoes (mass = m) be?????
h
m
Can you derive an equation for the
work done against gravity when I lift
the sack of potatoes?
So work done:
W = Fxd
If I do work against gravity I raise an object a
certain height (h) increasing its g.p.e. So:
W=Fxh
In order to lift the object I have to overcome the
objects weight (m x g), so the force has to be at
least equal to the weight. So:
W=mxgxh
But since the total amount of g.p.e gained must be equal to
the amount of energy transferred, or work done against
gravity therefore:
Work done against gravity = g.p.e
it follows that:
Easy
Write the equation to calculate work
done.
Medium
What is the work done if we apply a
1.2N force and we move 4 m in the
direction of force?
Hard
Thinking about energy transfers:
When you rub your hands together
what are the energy transfers?
Rearrange this equation for force
applied.
What is the work done if we apply a
7N force and we move 8 m in the
direction of the force?
When you boil a kettle what energy
transfers are happening?
Rearrange this equation for distance What distance is moved if we have
moved in the direction of force.
a 8N force and the work done is 90
J?
Walking up a flight of stairs, what
are the energy transfers?
What is the unit of force?
What is the distance moved if we
have a 70N force and work done is
8 J?
When you roll a ball down a hill what
are the energy transfers?
What is the unit of work done?
What force is required to move 7 m
if the work done is 9 J?
What is the work done when a force
of 5 N is applied to a ball and it
moves 80 m?
What is the unit of distance?
What work is done when we apply a
force of 5N and move in the
direction of the force 2 m?
What force is required to move 19 m What is the work done to a car if a
if the work done is 9 J?
force of 9 N is applied and it moves
7 km?
What force is required to move 7 m
if the work done is 21 J?
What is the work done to a person if
a force of 1.3N is applied and the
person moves 6m?
Easy
Write the equation to calculate work
done.
Medium
What is the work done if we apply a
1.2N force and we move 4 m in the
direction of force?
Hard
Thinking about energy transfers:
When you rub your hands together
what are the energy transfers?
Rearrange this equation for force
applied.
What is the work done if we apply a
7N force and we move 8 m in the
direction of the force?
When you boil a kettle what energy
transfers are happening?
Rearrange this equation for distance What distance is moved if we have
moved in the direction of force.
a 8N force and the work done is 90
J?
Walking up a flight of stairs, what
are the energy transfers?
What is the unit of force?
What is the distance moved if we
have a 70N force and work done is
8 J?
When you roll a ball down a hill what
are the energy transfers?
What is the unit of work done?
What force is required to move 7 m
if the work done is 9 J?
What is the work done when a force
of 5 N is applied to a ball and it
moves 80 m?
What is the unit of distance?
What work is done when we apply a
force of 5N and move in the
direction of the force 2 m?
What force is required to move 19 m What is the work done to a car if a
if the work done is 9 J?
force of 9 N is applied and it moves
7 km?
What force is required to move 7 m
if the work done is 21 J?
What is the work done to a person if
a force of 1.3N is applied and the
person moves 6m?
AIM: TO INVESTIGATE HOW
HARD WE CAN WORK AS A
CLASS!
Name
Mass
(kg)
Force
(N)
Distance
(m)
Work of
one lift
(J)
# of lifts in
1 min
Total work (J)
Mean Total Work for the group :
J
ARM CURLS
Force required = weight of object = mass (kg) x 10
distance
CLASS RESULTS FOR EACH
GROUP
Group
Mean Total
work (J)
Class Mean:
1
2
3
4
5
Conclusion:
Are we a
hardworking class?
J
LET’S SUMMARISE!
Discuss with the
person next to you
what you have
learned today then as
a pair write down what
you feel is the most
important thing you
learned.
Different Types
of Energy.
Heat/Thermal Energy
Anything with a temperature above
absolute zero (-273°C) has heat
energy. That means everything has
some heat energy. The hotter
something is the more heat it has.
Kinetic Energy
Anything that moves has kinetic energy.
Nuclear Energy
Released only from nuclear
reactions e.g. The sun and all of
the stars. Hydrogen bomb (Fusion)
Nuclear power plants and the
Atomic bomb. (Fission)
Sound Energy
Anything noisy gives off sound energy like vocal chords,
speakers and instruments.
Light Energy
Anything luminous gives off light energy, like the sun,
light bulbs , candles and glow worms.
Chemical Energy
Anything with stored energy which can be
released by a chemical reaction has chemical
energy, things like food fuels and batteries.
Electrical Energy
Electrical energy is very useful, because its easily converted
into other forms – wherever there's a current flowing there's
electrical energy.
Gravitational Potential Energy (GPE)
Anything above the ground has gravitational
potential energy i.e. anything that can fall, like
ski jumpers, aeroplanes and climbers.
Elastic Energy
Anything stretched, has elastic energy – things like rubber
bands, springs, knickers, elastic etc
Different types of energy
There are many different types of energy:
thermal
light
sound
elastic
gravitational
kinetic
electrical
chemical
nuclear
Can you think of
examples of each
type of energy?
Which type of energy?
Type
Heat
Kinetic (movement)
Nuclear
Sound
Light
Chemical
Electrical
Gravitational potential
Elastic potential
3 example sources
Type
3 example sources
Flows from hot objects to colder objects, e.g radiator, heater
and fire.
Anything that moves has kinetic energy., e.g football in mid
air, formula one car at speed, a cheetah.
Released only from nuclear reactions e.g. The sun and all of the
stars. Hydrogen bomb.
Nuclear power plants and the Atomic bomb
Anything noisy gives off sound energy like vocal chords,
speakers and instruments
Anything luminous gives off light energy, like the sun, light
bulbs , candles and glow worms.
Anything with stored energy which can be released by a
chemical reaction has chemical energy, things like food, fuels
and batteries.
Electrical energy is very useful, because its easily converted into
other forms – wherever there's a current flowing there's electrical
energy.
Anything above the ground has gravitational potential energy i.e. anything
that can fall, like ski jumpers, aeroplanes and climbers.
Anything stretched, has elastic energy – things like rubber bands,
springs, knickers elastic etc
Energy Transfer
Learning Objectives:
Identify the different types of Energy Transfer
Learning Outcomes:
All - Identified and recorded the different types of energy
Most – Discussed and explained their initial thoughts on
energy.
Some – Explaining why different objects emit more than one
type of energy.
Type
3 example sources
Heat
Flows from hot objects to colder objects, e.g radiator, heater
and fire.
Kinetic (movement)
Anything that moves has kinetic energy., e.g football in mid
air, formula one car at speed, a cheetah.
Nuclear
Released only from nuclear reactions e.g. The sun and all of the
stars. Hydrogen bomb.
Nuclear power plants and the Atomic bomb
Sound
Anything noisy gives off sound energy like vocal chords,
speakers and instruments
Light
Anything luminous gives off light energy, like the sun, light
bulbs , candles and glow worms.
Chemical
Anything with stored energy which can be released by a
chemical reaction has chemical energy, things like food, fuels
and batteries.
Electrical
Electrical energy is very useful, because its easily converted into
other forms – wherever there's a current flowing there's electrical
energy.
Gravitational potential
Anything above the ground has gravitational potential energy i.e. anything
that can fall, like ski jumpers, aeroplanes and climbers.
Elastic potential
Anything stretched, has elastic energy – things like rubber bands,
springs, knickers elastic etc
Which type of energy?
Energy transfer
Energy can be changed from one form to another.
For example:
 Chemical energy in food is
converted to thermal energy
and kinetic energy by our bodies.
 Gravitational energy in a ball is
converted to kinetic energy when
it falls to the ground.
What other energy transfers can you think of?
What
is
the
energy
transfer?
What energy transfer takes place in each device?
 burning match
 portable torch
 microphone
 radio
 television
 catapult
 mobile phone
 car
chemical to heat and light
chemical to heat and light
sound to electrical
electrical to sound and heat
electrical to sound and light and heat
elastic to kinetic and heat
chemical to sound and microwaves
(EM radiation) and heat
chemical to kinetic and sound and heat
In all these transfers the energy is not lost, it is conserved.
Energy cannot be destroyed or created.
Draw an energy transfer for... YOU!!
Sound
Chemical
Heat
Kinetic (movement)
Chemical
Elastic potential
Heat
Kinetic (movement)
Gravitational potential
Nuclear
Electrical
Sound
Light
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
DO NOW
1. Define resultant force
2. Describe what happens to an
object when the resultant force is
zero
3. If a car is moving forwards at a
force of 10N but friction acts on
the car at 4N, what is the resultant
force?
Extension
What does kinetic mean?
What does kinetic energy
mean?
What would the units be?
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
DO NOW
1. The sum of all the forces acting
on an object
2. Stationary or constant speed
3. 10 - 4 = 6N
Extension
movement
movement energy
Joules (J)
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
DO NOW
1. The sum of all the forces acting
on an object
2. Stationary or constant speed
3. 10 - 4 = 6N
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
• Kinetic energy is energy an object
has due to motion. It depends on
two things:
• Any ideas???
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
• Kinetic energy is energy an object
has due to motion. It depends on
two things:
• The objects mass
• The objects speed
• Kinetic energy tells us how much movement
energy something has.
• It doesn't just depend on how fast something is moving,
it also depends on mass.
• If a car and a lorry are both travelling at the same speed
the lorry will do much more damage if it hits something,
than if the car does.
• The lorry has more kinetic energy even though they are
both travelling at the same speed.
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
Formula triangle
TASK:
• Copy the
triangle – can
you rearrange
the equation?
• Add units and
name next to
symbol
V = √(2x KE)÷m
M = (2xKE) ÷ v2
RECALL: “Killer man-eating
squirrels!”
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
• Kinetic energy =
½ x mass x speed2
• Units of KE= joules (J)
• Mass= kg
• Speed= m/s
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
• A car with a mass of 500 kg is moving at a
speed of 12 m/s. How much kinetic energy
does it have?
EXAMPLE
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
• A car with a mass of 500 kg is moving at a
speed of 12 m/s. How much kinetic energy
does it have?
• ½ x 500 x (122) = 36,000 J
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
Calculate Kinetic Energy!
1. A car that travels at a speed of 20m/s and has a
mass of 1200kg
2. A year 11 pupil with a mass of 55kg swinging
back on their chair and falling off at 0.6m/s
3. A runner with a mass of 62kg running at a speed
of 0.8m/s
4. A tennis ball travelling at 46m/s with a mass of
58kg
5. A dog running across a field at a speed of
1.2m/s with a mass of 3.2kg
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
Calculate Kinetic Energy!
1. 600kg x 400m/s = 240,000J or 240KJ
2. 25kg x 0.36m/s = 9J
3. 31kg x 0.64m/s = 19.84J
4. 29kg x 2116m/s = 61,364J or 61.4KJ
5. 1.6kg x 1.44m/s = 2.304J
Peerassessment
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
Complete the
Tarsia
GPE=KE relationship
You will need to be able to rearrange!
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
Answer as many questions as you can
but you must end on your TAG
Kinetic Energy
Learning Objectives: Recall that a moving object has kinetic energy; State the
relationship between kinetic energy and gravitational potential energy; Apply and
rearrange the kinetic energy equation
1. It’s velocity is zero even though the object has mass. Multiplying the mass
by zero will result in zero
2. Car B has a greater velocity. KE =1/2mv2 so if they all have the same mass
the velocity will impact the KE. (Calculations of all 3 KEs needed)
3. 0.2 x400 =80J
4. Greater vlelocity = greater kinetic energy of the vehicle which will require
more force to stop completely from the brakes
5. 2x43200 = 86400/600 =square root answer = 12m/s
Answer as many questions as you can
but you must end on your TAG
Green = E-D
Yellow = C-B
Blue = A-A*
Why does a book on a
What is kinetic energy?
When a catapult is
shelf have zero kinetic
stretched and fired name
energy?
the energy transfers
An object weighs 2kg and
A car moves at 360KJ of
is fired horizontally at a
kinetic energy at a speed
speed of 5m/s. What is the of 30m/s. Calculate it’s
kinetic energy?
mass.
Why does a lorry have
more kinetic energy than a
mini?
State the equation for
Kinetic Energy and
rearrange this for Speed
and mass
Calculate the kinetic
What is the kinetic energy
energy of a van of mass
when a man lands if the
500kg moving at a speed
gravitational potential
of 12m/s
energy of him falling out of
a plane is 5642J?
Green = E-D
Yellow = C-B
Blue = A-A*
The energy an object
The book has mass
has due to it’s motion.
but it is not moving so
It depends on its mass
it has zero velocity
and speed.
Elastic potential
energy to kinetic
energy
25J
800kg
KE = 1/2mv2
M= (KE ÷ v2) x2
V = √ KE ÷ 1/2m
The lorry has more
mass so it has more
kinetic energy if the
mini is travelling at the
same velocity
36,000J
5642J
Sit in your allocated seat, get out your equipment and
write the title, LQ and date,
and begin the starter.
07 March 2022
Title: Efficiency
Learning Questions
 What is efficiency?
 Why is efficiency important?
Keywords: efficiency
Starter
“energy saving bulbs”
What does this mean?
Success criteria
• Define what
efficiency means.
Grade 4
Extension –
Grade 6
• Explain some ways in
which energy is
transferred wastefully
by mechanical
processes.
• Explain some ways of
reducing unwanted
energy transfers in
mechanical
processes.
Grade 8
What is efficiency?
The efficiency of a device is a way of saying how good it is at
transferring energy as a useful output.
The efficiency of a device is given as a number between 0
and 1. The higher the number, the more efficient the device.
0
0.5
1.0
Wastes all
energy
transferred
to it.
Almost all energy
transferred into
useful energy (no
device 100%
efficient).
0%
50%
100%
What is efficiency?
Efficiency is a measure of how much useful
energy we get out compared to what goes in
Energy In
How do we calculate efficiency?
Efficiency is calculated using this formula:
efficiency =
(useful energy transferred by the device)
(total energy supplied to the device)
We can write this more briefly as:
efficiency =
useful energy out
total energy in
Things to remember when you are using the formula:
• the useful energy transferred by the device is
always less than the total energy supplied
• the efficiency of a device can never be greater
than 1.
If you calculate an efficiency greater than 1, or
calculate an amount of useful energy that is greater
than the total energy transferred, you have either
substituted numbers into the formula incorrectly or
you have made a mistake in your calculation!
Progress Questions
1 A device transfers 30 J of useful energy every
second. The total energy transferred to the device
each second is 50 J. Calculate its efficiency.
(useful energy transferred by the device)
efficiency =
(total energy supplied to the device)
= 30 J
50 J
= 0.6
Check - is the efficiency less than 1?

Progress Questions
2 A light bulb transfers 90 J of energy by heating every
second. 100 J of energy is transferred to the light bulb
every second by electricity. Calculate its efficiency.
The energy transferred by heating is wasted energy in a
light bulb.
Energy is conserved, so:
total energy transferred = useful energy + wasted energy
100 J = ?? + 90 J
useful energy transferred = 10 J
Continued
Progress Questions
2 A light bulb transfers 90 J of energy by heating every
second. 100 J of energy is transferred to the light bulb
every second by electricity. Calculate its efficiency.
(useful energy transferred by the device)
efficiency =
(total energy supplied to the device)
= 10 J
100 J
= 0.1
Check - is the efficiency less than 1?

Progress Questions
3 Calculate the efficiency of this heater.
Continued
Efficiency practical
• What energy
transfers take
place when a ball
bounces?
• Does a ball lose
energy when it
bounces?
Ball efficiency
Drop
height
(cm)
20
40
60
80
100
Bounce height (cm)
Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
Average
Uncertainty
How certain are you of your results?
• Was it easy to see the bounce height
ACCURATELY?
• What will this mean for your results?
• Did you have any anomalous results?
Drop
height
(cm)
Bounce height (cm)
Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
20
11
10
7
9
11
40
17
24
22
19
21
60
30
32
34
29
29
80
55
57
59
54
56
100
80
81
83
81
80
Average
Uncertaint
y
Which of my results do you think are the most
accurate and why?
• Add error bars to
your graphs.
– Does you line of best
fit pass through all of
your error bars?
• Calculate your
gradient. This will
be the efficiency of
the ball.
Progress Questions
3 Calculate the efficiency of the heater. 100 J is supplied
to the heater each second. 1 J is transferred by light,
4 J by sound and 95 J by heating.
(useful energy transferred by the device)
efficiency =
(total energy supplied to the device)
= 95 J
100 J
= 0.95
Check - is the efficiency less than 1?

Progress Questions
4 A device transfers 20 J of useful energy every
second. It has an efficiency of 0.8. How much energy is
transferred to the device each second?
total energy =
useful energy
efficiency
= 20 J
0.8
= 25 J
Check - is the total energy greater than the useful
energy transferred?

Progress Questions
5 A light bulb transfers a total of 20 J of energy every
second. Its efficiency is 0.45. How much energy does it
transfer by light each second?
useful energy = efficiency x total energy
= 0.45 x 20 J
=9J
Check - is the useful energy less than the total
energy transferred?

Progress Checker
1: Your muscles waste about 75 J of energy for every 25 J
they convert into movement. How efficient are your
muscles?
2: An electric fan has an efficiency of 80%. If it produces
120 W of useful kinetic energy in the air, how much
power is it using?
Swap & Mark
1: Total energy transferred = 25 J + 75 J = 100 J.
Efficiency = 25 J/100 J = 0.25 (or 25%).
2: Total power in = useful power out/efficiency =
120 W/0.8 = 150 W.
Progress– plenary task (6 mark
question)
Grade 4
2 marks
Grade 6
4 marks
Grade 8
6 marks
• The table gives data about two types of light bulb people
may use in their homes.
• Both types of light bulb produce the same amount of light.
• Evaluate, in terms of cost and energy efficiency, the use of the
two types of light bulb.
• To gain full marks you must compare both types of light bulb
and conclude which light bulb would be the best to use.
Level 1 (1-2 marks)
• There is a basic comparison of either a cost aspect or
an energy efficiency aspect.
Level 2 (3-4 marks)
• There is a clear comparison of either the cost aspect
or energy efficiency aspect OR a basic comparison of
both cost and energy efficiency aspects
Level 3 (5-6 marks)
• There is a detailed comparison of both the cost
aspect and the energy efficiency aspect.
• For full marks the comparisons made should support
a conclusion as to which type of bulb is preferable
Cost
• halogen are cheaper to buy
(simply giving cost figures is
insufficient)
• 6 halogen lamps cost the same as
one LED
• LEDs last longer
• need to buy 18 / more halogen
lamps to last the same time as
one LED
• 18 halogens cost £35.10
• costs more to run a halogen than
LED
• LED has lower maintenance cost
(where many used, eg large
departmental store lighting)
SWAP & MARK
Energy efficiency
• LED works using a
smaller current
• LED wastes less energy
• LEDs are more efficient
• LED is 22% more energy
efficient
• LED produces less heat
• LED requires smaller
input (power) for same
output (power)
Progression questions
1. What does efficiency mean?
2. How do we calculate the efficiency of an
energy transfer?
3. How can we reduce unwanted energy
transfers in machines?
Plenary
What can you do now?
define what efficiency means
recall and use the formula for calculating
energy efficiency
explain some ways in which energy is
transferred wastefully by mechanical
processes
explain some ways of reducing unwanted
energy transfers in mechanical processes.