1. EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 4-3 Work and Power
Edexcel IGCSE Physics pages 142 to 149
June 17th 2012
All content applies for Triple & Double Science

2. Edexcel Specification
Section 4: Energy resources and energy transfer
c) Work and power
know and use the relationship between work, force and distance moved in the direction of the force:
work done = force × distance moved W = F × d
understand that work done is equal to energy transferred
know and use the relationship:
gravitational potential energy = mass × g × height
GPE = m × g × h
kinetic energy = ½ × mass × speed2 KE = ½ × m × v2
understand how conservation of energy produces a link between gravitational potential energy, kinetic energy and work
describe power as the rate of transfer of energy or the rate of doing work
use the relationship between power, work done (energy transferred) and time taken:
power = work done / time taken P = W / t

3. Work and energy
When a force causes a body to move through a distance, energy is transferred and work is done.
work done = energy transferred.
Both work and energy are measured in joules (J). 3

4. Work and friction
Work done against frictional forces is mainly transformed into heat.
Rubbing hands together causes them to become warm.
Brakes pads become hot if they are applied for too long. In this case some of the car’s energy may also be transferred to sound in the form of a ‘squeal’ 4

5. The work equation
work done = force applied × distance moved in the direction of the force
W = F x d
work, W is measured in joules (J)
force, F is measured in newtons (N)
distance, d is measured in metres (m) 5

6. W F d also: force = work done ÷ distance moved and:
distance = work done ÷ force W F d 6

7. Question 1
Calculate the work done when a force of 5N moves through a distance of 3m.
W = F x d
= 5N x 3m
work = 15 J 7

8. Question 2
Calculate the work done when a force of N moves through a distance of 40cm.
W = F x d
= 6 N x 40 cm
= 6 N x 0.40 m
work = 2.4 J 8

9. Question 3
Calculate the value of the force required to do 600J of work over a distance of 50m.
W = F x d
becomes:
F = W ÷ d
= 600 J ÷ 50 m
force = 12 N 9

10. Question 4
Calculate the distance moved by a force of 8N when it does 72J of work.
W = F x d
becomes:
d = W ÷ F
= 72 J ÷ 8 N
distance moved = 9 m 10

11. Question 5
Calculate the work done by a child of weight 300N who climbs up a set of stairs consisting of 12 steps each of height 20cm.
W = F x d
The child must exert an upward force equal to its own weight.
Therefore: force = 300N
This force is exerted upwards and so the distance must also be measured upwards.
= (12 x 20cm)
= 2.4m
therefore:
work = 300 N x 2.4 m
work = 720 J 11

12. Question 6
Calculate the work done by a person of mass 80kg who climbs up a set of stairs consisting of 25 steps each of height 10cm.
W = F x d
the person must exert an upward force equal their weight
the person’s weight = (80kg x 10N/kg) = 800N
the distance moved upwards equals (10 x 25cm) = 2.5m
work = 800 N x 2.5 m
work = 2000 J 12

13. Complete Answers work force distance J 50 N 3 m 800 J N 20 m 500 JkJ
4000 N
2 m
2 MJ
3.03 N
5 km
150 40 2 80
400 13

14. Choose appropriate words to fill in the gaps below:
Work is done when a _______ moves through a distance.
The amount of _______ transferred is also equal to the work done. When a car brakes energy is transformed to ______.
Work done is ______ to the force _________ by the distance moved in the __________ of the force. The work done is measured in ______ if the force is measured in newtons and the _________ in metres.
force
energy
heat
equal
multiplied
direction
joules
distance
WORD SELECTION:
energy
direction
force
equal
multiplied
distance
heat
joules 14

15. Gravitational potential energy
Gravitational potential energy (GPE) is the energy stored in an object when work is done in moving the object upwards.
GPE = mass x g x height
GPE = m x g x h
GPE is measured in joules (J)
mass, m is measured in kilograms (kg)
gravitational field strength, g is measured
in newtons per kilogram (N/kg)
height, h is measured in metres (m) 15

16. Question 1
Calculate GPE gained when a weightlifter lifts a mass of 120kg up by 2.5m.
(g = 10N/kg)
GPE = m x g x h
= 120kg x 10N/kg x 2.5m
GPE = J 16

17. Question 2
Calculate the gravitational potential energy gained by a student of mass 70kg climbing a flight of stairs of height 4m.
GPE = m x g x h
= 70kg x 10N/kg x 4m
GPE = J 17

18. kinetic energy = ½ x mass x (speed)2
Kinetic energy is the energy possessed by a body because of its speed and mass.
kinetic energy = ½ x mass x (speed)2
KE = ½ x m x v2
kinetic energy, KE is measured in joules (J)
mass, m is measured in kilograms (kg)
speed, v is measured in metres per second (m/s) 18

19. Question 1
Calculate the kinetic energy of a car of mass 1000kg moving at 5 m/s.
KE = ½ x m x v2
= ½ x 1000kg x (5m/s)2
= ½ x x 25
= x 25
kinetic energy = J 19

20. Question 2
Calculate the kinetic energy of a child of mass 60kg moving at 3 m/s.
KE = ½ x m x v2
= ½ x 60kg x (3m/s)2
= ½ x 60 x 9
= 30 x 9
kinetic energy = 270 J 20

21. Question 3
Calculate the kinetic energy of a apple of mass 200g moving at 12m/s.
KE = ½ x m x v2
= ½ x 200g x (12m/s)2
= ½ x kg x 144
= x 144
kinetic energy = J 21

22. Question 4
Calculate the mass of a train if its kinetic energy is 2MJ when it is travelling at 4m/s.
KE = ½ x m x v2
2MJ = ½ x mass x (4m/s)2
J = ½ x mass x 16
= 8 x mass
÷ 8 = mass
mass = kg 22

23. Question 5
Calculate the speed of a car of mass 1200kg if its kinetic energy is J.
KE = ½ x m x v2
15 000J = ½ x 1200kg x (speed)2
= x (speed)2
÷ 600 = (speed)2
25 = (speed)2
speed = 25
speed = 5 m/s 23

24. Question 6
Calculate the speed of a ball of mass 400g if its kinetic energy is 20J.
KE = ½ x m x v2
20J = ½ x 400g x (speed)2
20 = ½ x kg x (speed)2
20 = x (speed)2
20 ÷ = (speed)2
100 = (speed)2
speed = 100
speed = 10 m/s 24

25. Complete Answers kinetic energy mass speed J 4 kg 2 m/s 27 J kg 3 m/skJ
200 kg
8 m/s
3.2 J
3.03g
4 m/s 8 6 5
6.4
400 25

26. Falling objects
If there is no significant air resistance then conservation of energy results in gravitational potential energy being converted into kinetic energy as an object falls.
gain in KE = loss of GPE m
gpe = mgh
ke = 0 h
gpe = ke
½ h v1
gpe = ½ mgh
ke = ½ mv12
gpe = 0 v2
ke = ½ mv22
ke = mgh 26

27. object reaches lowest point
Graphs of GPE and KE
Energy
Time
TOTAL ENERGY = GPE + KE
GPE KE
object dropped
object reaches lowest point 27

28. Question
A child of mass 40kg climbs a wall of height 3m and then steps off. Calculate the speed at which the child reaches the bottom of the wall.
Child’s initial gravitational potential energy:
GPE = m x g x h
= 40kg x 10N/kg x 3m
GPE = J
If air resistance is insignificant then all of this GPE is converted into kinetic energy
KE = ½ x m x v2
1200 J = ½ x 40kg x (speed)2
= 20 x (speed)2
1 200 ÷ 20 = (speed)2
60 = (speed)2
speed = 60
speed = 7.75 m/s 28

29. Choose appropriate words to fill in the gaps below:
Gravitational ________ energy is the energy stored when an object is lifted ________. This energy is released when the object _____ back to its initial position.
Kinetic energy is the energy possessed by an object due to its _______ and mass. If the mass of an object is ________ its kinetic energy doubles. If the speed is doubled the kinetic energy will increase by ______ times.
When a __________ object is released gravitational potential energy is converted into _________ energy.
potential
upwards
falls
speed
doubled
four
raised
kinetic
WORD SELECTION:
falls
speed
four
kinetic
potential
upwards
doubled
raised 29

30. Power (P) Power is a measurement of how quickly work is done.
power = work done
time taken
P = W t
power, P is measured in watts (W)
work done, W is measured in joules (J)
time, t is measured in seconds (s)
One watt is the same as one joule per second. 30

31. Power is also equal to how quickly energy is transformed from one form to another.
power = energy change
time
P = E t 31

32. Question 1
Calculate the power of a motor that exerts a force of 40N over a distance of 2m for 10seconds.
W = F s
= 40 N x 2 m
work done = 80 J
P = W / t
= 80J / 10 s
power = 8.0 W 32

33. Question 2 gain in GPE = m g h = 50 kg x 10 N/kg x 3 m = 1500 J
Calculate the power of an electric motor that lifts a mass of 50 kg upwards by 3.0 m in 20 seconds.
g = 10 N/kg
gain in GPE = m g h
= 50 kg x 10 N/kg x 3 m
= 1500 J
P = E / t
= 1500 J / 20 s
power = 75 W 33

34. Complete: Answers 600 J 5 W 440 J 20 s 28 800 J 28 800 J 2500 J 50 W
energy transfer
work done
time
power
600 J
120 s
5 W
440 J
20 s
22 W J
2 hours
4 W
2.5 mJ
2.5 kJ
50 s
50 W
600 J
5 W
440 J
20 s J J
2500 J
50 W 34

35. Measuring a person’s power
1. Measure the weight, W of person using weighing scales.
2. Measure the time taken for the person to run up a flight of stairs of height, h
3. Work done
= weight x height
= W x h
= W x n x s
4. Power of the person
= work done / time taken
= (W x n x s) / t
total stairs height, h
= n x s
stairs of n steps s
person of weight, W 35

36. Example calculation Weight of person, W = 800N
Time taken, t = 3.0 seconds
Stairs:
number of steps, n = 12
height of step = 0.20m
total stair height, h
= 12 x 0.20m = 2.4m
Work done
= weight x height
= 800N x 2.4m = 1920J
Power = 1920J / 3.0s
= 640W
total stairs height, h
= n x s
stairs of n steps s
person of weight, W 36

37. Choose appropriate words to fill in the gaps below:
Power is a measure of how ___________ a device does work. Power is equal to work done in _________divided by the time taken.
The _________ of a device is also equal to the rate at which a device transforms ___________ from one form to another.
Power is measured in _________, symbol W.
A one kilowatt motor will perform one ____________ joules of work every __________.
quickly
joules
power
energy
watts
thousand
second
WORD SELECTION:
second
quickly
power
thousand
watts
joules
energy 37

38. Work and Power Notes questions from pages 142 to 149
Give the equation for work done, state the units used and give an example calculation.
Give the equation for kinetic energy, state the units used and give an example calculation.
Give the equation for gravitational potential energy, state the units used and give an example calculation.
Explain how gravitational potential and kinetic energy vary as an object falls. You should copy all the diagrams on page 146 as part of your answer.
Give the equation for power, state the units used and give an example calculation.
Explain how the power of a person can be measured using a flight of stairs.
Answer the questions on page 149.
Verify that you can do all of the items listed in the end of chapter checklist on page 149.

39. Online Simulations Work (GCSE) - Powerpoint presentation by KT
Kinetic Energy (GCSE) - Powerpoint presentation by KT
Gravitational Potential Energy (GCSE) - Powerpoint presentation by KT
Bouncing ball with different surfaces showing KE & PE - Freezeway.com
Energy Skate Park - Colorado - Learn about conservation of energy with a skater dude! Build tracks, ramps and jumps for the skater and view the kinetic energy, potential energy and friction as he moves. You can also take the skater to different planets or even space!
Rollercoaster Demo - Funderstanding
Energy conservation with falling particles - NTNU
Ball rolling up a slope - NTNU
Pulley System - Fendt
Bicycle gear distance multiplier demonstration - Freezeway.com
BBC AQA GCSE Bitesize Revision:
Work, force and distance
Potential and kinetic energy
Kinetic energy equation