1. Energy and Forces doing Work
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Energy and Forces doing Work
2016 EdExcel GCSE Physics
Topic 8
W Richards
The Weald School

2. Gravitational potential
The 9 types of energy
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Type
3 example sources
Type
3 example sources
Heat
Kinetic (movement)
Nuclear
Sound
Light
Chemical
Electrical
Gravitational potential
Elastic potential

3. The Laws of Physics
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There are many laws of physics, but one of the most important ones is:
Energy cannot be created or destroyed, it can only be converted from one form to another

4. Electricity Light + heat
Energy changes
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To describe an energy change for a light bulb we need to do 3 steps:
1) Write down the starting energy:
3) Write down what energy types are given out:
2) Draw an arrow
Electricity Light + heat
What are the energy changes for the following…?
An electric fire
A rock about to drop
An arrow about to be fired

5. Energy Transfer Examples
Look at the following objects and write down what their energy changes are:
a) Electric Light Bulb
b) Battery
c) Hairdryer
d) Candle Burning
e) Model Car (Wind-up)
f) Dynamo
g) Yo-Yo
h) Radio
i) Kettle
j) Dropping a Golf Ball
For each of these objects which stores of energy have gone up and which have gone down?

6. Work done = Force x distance moved
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When any object is moved around work will need to be done on it to get it to move (obviously).
We can work out the amount of work done in moving an object using the formula:
Work done = Force x distance moved
in J in N in m W d F

7. Example questions 25J 20J, GPE 4m, KE 50N 2MJ
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Amy pushes a book 5m along the table with a force of 5N. She gets tired and decides to call it a day. How much work did she do?
Jodie lifts a laptop 2m into the air with a force of 10N. How much work does she do? What type of energy did the laptop gain?
Ronnie does 200J of work by pushing a wheelbarrow with a force of 50N. How far did he push it? What type of energy did the wheelbarrow gain?
Julian cuddles his cat and lifts it 1.5m in the air. If he did 75J of work how much force did he use?
Travis drives his car 1000m. If the engine was producing a driving force of 2000N how much work did the car do?
25J
20J, GPE
4m, KE
50N
2MJ

8. Gravitational Potential Energy
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To work out how much gravitational potential energy (GPE) an object gains when it is lifted up we would use the simple equation…
GPE = mass x grav. field strength x Change in height
(Joules) (newtons) (N/Kg) (metres)
GPE H mg

9. Some example questions…
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How much gravitational potential energy have the following objects gained?:
A brick of mass 1kg lifted to the top of a house (10m),
A 1,000kg car lifted by a ramp up to a height of 2m,
A 70kg person lifted up 50cm by a friend.
How much GPE have the following objects lost?:
A 0.2kg football dropping out of the air after being kicked up 30m,
A 0.05kg egg falling 10m out of a bird nest,
A 1,000kg car falling off its 200cm ramp.
100J
20KJ
350J
60J 5J
20KJ

10. Kinetic energy = ½ x mass x speed squared
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Any object that moves will have kinetic energy.
The amount of kinetic energy an object has can be found using the formula:
Kinetic energy = ½ x mass x speed squared
in J in kg in m/s
KE = ½ mv2

11. Example questions
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Lydia drives her car at a speed of 30m/s. If the combined mass of her and the car is 1000kg what is her kinetic energy?
Sam rides her bike at a speed of 10m/s. If the combined mass of Sam and her bike is 80kg what is her kinetic energy?
Josh is trying to catch a bus and is running at 3m/s. If he has a mass of 60kg how much kinetic energy does he have?
A 2000kg car is being driven at a speed of 10m/s. If it doubles its speed to 20m/s what happens to the car’s kinetic energy?
450,000J
4000J
270m/s
Increases from 100,000J to 400,000J

12. Example questions (higher)
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Steve is running away from the police and has 100J of kinetic energy. If he is running at 2m/s what is his mass?
James is driving in his car and has a combined mass of 1200kg. If he has 540KJ of kinetic energy what speed is he driving at?
Dave is running and has a kinetic energy of 750J. If his mass is 60kg how fast is he running?
Stuart is spotted walking around Tescos. If he has a kinetic energy of 150J and he’s walking at a pace of 2m/s what is his mass?
50kg
30m/s
5m/s
75kg

13. Recap: The Laws of Physics
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Recall one of the most important laws of Physics:
Energy cannot be created or destroyed, it can only be converted from one form to another

14. Conservation of Energy
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In any energy change there is ALWAYS some “waste” energy:
e.g. a light bulb:
Electricity Light heat
In this example HEAT is wasted and it is transferred to the surroundings, becoming very difficult to use.
Describe the following energy changes and state the “waste” energy or energies:
A vacuum cleaner
A TV
A dynamo/generator

15. Examples of wasted energy
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Clearly, not all of the energy from the fuel put into this car will become movement energy. Where is energy wasted in this car? Where does this energy go?
The bulb on the left is an “energy efficient” one. Why is the bulb on the right wasting energy?

16. In other words, 1 Watt = 1 Joule per second
Energy and Power
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POWER is defined as “the rate of doing work” or “the rate of transferring energy” and is measured in Watts.
In other words, 1 Watt = 1 Joule per second
Power = work done or energy
time E T P
W = Work done (in joules)
P = Power (in watts)
T = Time (in seconds)

17. Some example questions
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What is the power rating of a light bulb that transfers 120 joules of energy in 2 seconds?
What is the power of an electric fire that transfers 10,000J of energy in 5 seconds?
Georgia runs up the stairs in 5 seconds. If she transfers 1,000,000J of energy in this time what is her power rating?
How much energy does a 150W light bulb transfer in a) one second, b) one minute?
Brad’s brain needs energy supplied to it at a rate of 40W. How much energy does it need during a 50 minute (3000 second) physics lesson?
Gabriel’s brain, being more intelligent, only needs energy at a rate of about 20W. How much energy would his brain use in a normal day?
60W
2KW
0.2MW
150J, 9KJ
120KJ
1.73MJ

18. Efficiency
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Efficiency is a measure of how much USEFUL energy you get out of an object from the energy you put INTO it.
For example, consider a TV:
Light (80J)
Electrical
Energy (200J)
Sound (40J)
Heat (?)
Efficiency = Useful energy out
Energy in
x100%

19. Some examples of efficiency…
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5000J of electrical energy are put into a motor. The motor converts this into 100J of movement energy. How efficient is it?
A laptop can convert 400J of electrical energy into 240J of light and sound. What is its efficiency? Where does the rest of the energy go?
A steam engine is 50% efficient. If it delivers 20,000J of movement energy how much chemical energy was put into it? 2%
60%, given out to the surroundings as heat
40,000J