Under development.

1. Energy, Work and Power
Canadian Academy, Kobe
draft presentation under revision staylor@canacad.ac.jp

2. Energy, Work & Power
Unit Question: “How is energy used to produce change?”
Areas of interaction:
Human ingenuity
We design machines to maximise efficiency
Criterion Assessment Tasks

3. Energy, Work & Power
Assessment Statements
• Define energy
• Identify the form(s) of energy possessed by an object or system
• Distinguish between kinetic and potential energy
• Compare the relative quantities of a form of energy possessed by a set of objects.
• Define work
• Identify how work affects the quantity of energy in an object
• Define power
• Apply power to the time and work needed to complete a task.
• State the SI or commonly-used units for work, energy and power
[including Joules (J), Watts (W), Calories, kcal, kiloWatt hours (kWh)]
• Define efficiency
• Apply efficiency to the energy or power needed to complete a task.
• State the principle of conservation of energy
• Apply work, power and conservation of energy to discuss energy transfers and
transformations in a closed system.

4. お元気ですか。

5. お元気ですか。
O-genki desu-ka?
“energetic”
Energy is the ability to do work.
Oh yeah, Grade 10.

6. Energy is the ability to do work.
Unit: joules (J)
Many types, including:
• Mechanical, Elastic, Gravitational, Kinetic
• Chemical, Thermal, Sound, Luminous
• Nuclear, Radiant, Magnetic, Electrical
• Potential (stored)
e.g. gravitational potential e.g chemical potential
e.g. elastic potential energy
http://www.clker.com

7. Energy is the ability to do work.
Potential energy can be converted to kinetic energy
e.g. gravitational potential
As the cart rolls down the hill,
gravitational potential energy is
converted to kinetic energy.

8. Energy is the ability to do work.
Unit: joules (J)
Many types, including:
• Mechanical, Elastic, Gravitational, Kinetic
• Chemical, Thermal, Sound, Luminous
• Nuclear, Radiant, Magnetic, Electrical
• Potential (stored)
e.g. gravitational potential
The action of a force to cause
displacement of an object.
Work (J) = force (N) x distance (m)
1 Joule = 1N x 1m

9. Get up the stairs! Use chemical potential energy to generate mechanical
energy to increase your gravitational potential energy!
Devise a method to determine how much work is done by each of the
members of your group in getting up the stairs. Consider vertical movement
only for now and the force as their weight. Show your working.
Extend your method to calculate the power of each
person as they ascend the stairs.
What variable can they change to be
‘more powerful’?
What about going down
the stairs?
Is running upstairs more
work that walking?
http://timeandplace.martenhendriks.nl/2008/07/july-21-stairs-drawing-marten-hendriks.html

10. Who has done the most work?
300kg 500N 500N
no movement 100kg
6m
10s
5s
http://www.clker.com/clipart-man-push.html

11. Who has done the most work?
Work = Force x Distance
300kg 500N 500N
no movement 100kg
6m
10s
5s
http://www.clker.com/clipart-man-push.html

12. Who has done the most work?
Work = Force x Distance
Work = 500N x 0m Work = 500N x 6m
= 0J = 3000J (3kJ)
300kg 500N 500N
no movement 100kg
6m
10s
5s
http://www.clker.com/clipart-man-push.html

13. Now who has done the most work?
Work = F x d
3600N
= _____J (__kJ)
300kg
Work = 500N x 6m
= 3000J (3kJ)
60s 10m
500N
100kg
6m
5s
http://www.clker.com/clipart-man-push.html

14. Now who has done the most work?
Work = 3600N x 10m
3600N
= 36,000J (36kJ)
300kg
Work = 500N x 6m
= 3000J (3kJ)
60s 10m
500N
100kg
6m
5s
http://www.clker.com/clipart-man-push.html

15. What is the gain in gravitational potential
energy of the rock, relative to the ground?
3600N
ug = weight x height
potential energy
due to gravity
300kg
60s 10m
http://www.clker.com/clipart-man-push.html

16. What is the gain in gravitational potential
energy of the rock, relative to the ground?
ug = weight x height
potential energy
due to gravity
300kg = 3000N x 10m
= 30,000J (30kJ)
10m
http://www.clker.com/clipart-man-push.html

17. Energy storage in a ball
Determine the type and magnitude of
energy stored in the ball held at 1m
height.
How much work is being done when you
lift the ball as high as you can?
How much work is being done when you
drop the ball to the ground?
Why doesn’t the ball bounce
back to the original starting
position?

18. Where did the rest of the energy go?
Work = 36,000J (36kJ)
ug = 30,000J (30kJ)
3600N
300kg
10m
http://www.clker.com/clipart-man-push.html

19. Energy is neither created
nor destroyed.
It can be transferred from one
object to another or transformed
from one form to another.
Law of conservation of energy.
(The First Law of Thermodynamics)

20. Where did the rest of the energy go?
Work = 36,000J (36kJ)
ug = 30,000J (30kJ)
3600N
Energy is neither created nor destroyed.
300kg
It can be transferred from one object to another or
transformed from one form to another.
Electrical energy
10m
http://www.clker.com/clipart-man-push.html

21. Where did the rest of the energy go?
Work = 36,000J (36kJ)
ug = 30,000J (30kJ)
3600N
Energy is neither created nor destroyed.
300kg
It can be transferred from one object to another or
transformed from one form to another.
Electrical energy
Mechanical energy
10m sound energy thermal energy
Kinetic energy
sound energy thermal energy
Gravitational
sound energy thermal energy
potential energy
http://www.clker.com/clipart-man-push.html

22. What energy transfers and transformations are taking place?
Energy In Energy Out
electrical luminous, sound, radiant, thermal

23. So… can an object do negative work?
When force causes a displacement, work (energy) is positive.
When force hinders a displacement, work (energy) is negative.
When force results in no displacement, there is no work.
Work is not a vector – but Force and displacement are.
Pushing the rock Pushing the rock up the
Holding the rock
up the hill hill – but the rock keeps
steady on the hill
rolling down
d
Aargh!

24. How much work is done by the man?
considers only the active force
600N
FN
1000N
100kg
50m
Ffr Fapplied
200N 600N
Fg
200N
1000N

25. How much work is done by the man?
considers only the active force
600N
FN
1000N
100kg
50m
Ffr Fapplied
200N 600N
Fg
200N
1000N
Work = 600N x 50m
= 30,000J (30kJ)

26. How much work is done by friction?
600N
FN
1000N
100kg
50m
Ffr Fapplied
200N 600N
Fg
200N
1000N

27. How much work is done by friction?
600N
FN
1000N
100kg
50m
Ffr Fapplied
200N 600N
Fg
200N
1000N
Work = -200N x 50m
= - 10,000J (-10kJ)

28. How much work is done by gravity?
600N
FN
1000N
100kg
50m
Ffr Fapplied
200N 600N
Fg
200N
1000N

29. How much work is done by gravity?
600N
FN
1000N
100kg
50m
Ffr Fapplied
200N 600N
Fg
200N
1000N
Work = 1000N x 0m
= 0J (0kJ)

30. Fail. The crane operator was not using his seatbelt. His mass (before impact) is 80kg.
Determine:
1. His ug relative to the ground (before).
2. His ug relative to the rock (before).
300kg
1. The work done by gravity (after).
Assume weight = force of gravity.
16m 10m
http://www.clker.com/clipart-man-push.html

31. Fail. The crane operator was not using his seatbelt. His mass (before impact) is 80kg.
Determine:
1. His ug relative to the ground (before).
ug = weight x height = 800N x 16m = 12,800J
2. His ug relative to the rock (before).
300kg
1. The work done by gravity (after).
Assume weight = force of gravity.
16m 10m
http://www.clker.com/clipart-man-push.html

32. Fail. The crane operator was not using his seatbelt. His mass (before impact) is 80kg.
Determine:
1. His ug relative to the ground (before).
ug = weight x height = 800N x 16m = 12,800J
2. His ug relative to the rock (before).
300kg ug = 800N x 6m = 4,800J
1. The work done by gravity (after).
Assume weight = force of gravity.
16m 10m
http://www.clker.com/clipart-man-push.html

33. Fail. The crane operator was not using his seatbelt. His mass (before impact) is 80kg.
Determine:
1. His ug relative to the ground (before).
ug = weight x height = 800N x 16m = 12,800J
2. His ug relative to the rock (before).
300kg ug = 800N x 6m = 4,800J
1. The work done by gravity (after).
Assume weight = force of gravity.
Work = F x d = 800N x 16m = 12,800J
16m 10m
http://www.clker.com/clipart-man-push.html

34. Energy is the ability to do work.
Unit: joules (J)
Many types, including:
• Mechanical, Elastic, Gravitational, Kinetic
• Chemical, Thermal, Sound, Luminous
• Nuclear, Radiant, Magnetic, Electrical
• Potential (stored)
e.g. gravitational potential
The action of a force to cause
When force causes a displacement, work (energy) is positive. displacement of an object.
When force hinders a displacement, work (energy) is negative.
When force results in no displacement, there is no work. Work (J) = force (N) x distance (m)
1 Joule = 1N x 1m
Power is the
rate of doing work or using energy.
work done (J) energy used (J)
Power (W) = =
1 Watt = 1J in 1s time (s) time (s)

35. Who has the most power?
a different guy work done (J)
Power (W) =
1 Watt = 1J in 1s time (s)
3600N
Crane =
300kg
Man =
60s 10m
500N
100kg
6m
5s
http://www.clker.com/clipart-man-push.html

36. Who has the most power?
work done (J)
Power (W) =
1 Watt = 1J in 1s time (s)
3600N
Crane = 36,000J =
60s
300kg
Man = 3000J =
5s
60s 10m
500N
100kg
6m
5s
http://www.clker.com/clipart-man-push.html

37. Who has the most power?
work done (J)
Power (W) =
1 Watt = 1J in 1s time (s)
3600N
Crane = 36,000J = 600W
60s
300kg
Man = 3000J = 600W
5s
60s 10m
500N
100kg
6m
5s
http://www.clker.com/clipart-man-push.html

38. Quick Quiz:
1. Define energy – what is its unit?
1. Define work – what is its unit?
1. Define power – what is its unit?
1. Name and describe as many forms of energy as you can.

39. What do you think?
Running 1km is harder, so is
more work than walking.
They are both the same
amount of work.
Walking takes longer, so is
more work.
Clipart people from: http://www.clker.com/

40. Elastic Bands & Bouncy Balls
Investigate one factor which my affect the motion of an elastic band or bouncy ball.
Resources here: http://goo.gl/XYYB9
“How does ________
affect ______?”
Elastic band finger gun from http://www.the-rubber-band.com/hand_shooting.php Bouncy ball: http://goo.gl/B3rYW

41. Balloon rockets
What forms of energy are evident when:
• The balloon is inflated?
• The balloon is let go?
• The balloon has stopped (feel it)?
thermal
electrical elastic
____ potential
kinetic mechanical
nuclear magnetic radiant
sound luminous
Review:
• Free body diagram for the balloon.
• Free body diagram for the straw.
• Reaction force pair for the movement.
http://www.education.com/science-fair/article/volume-air-far-balloon-rocket-travels/

42. Balloon rockets
What forms of energy are evident when:
• The balloon is inflated? elastic potential
• The balloon is let go? kinetic sound elastic
• The balloon has stopped (feel it)? thermal
Calculate the work done by the balloon.
Assume a mean force of 0.5N
Calculate the power of the balloon.
http://www.education.com/science-fair/article/volume-air-far-balloon-rocket-travels/

43. Balloon rockets: Blog Post
Assessed for Criteria C, B and F. Work in teams to run the rockets and make the videos, but
complete you own work on the blog post. This is in place of a unit test.
Instructions and task-specific clarifications: http://goo.gl/D9lLO
Quick notes on annotating YouTube videos: http://goo.gl/azNRc

44. is the ratio of useful work out from the total
Efficiency amount of work done, as a percentage.*
Useful work out (J)
Efficiency (%) = x 100
Total work done (J)
How is efficiency affected by energy transfers
and transformations in the system?
How can efficiency of a machine or system be
maximised?
*http://www.a-levelphysicstutor.com/m-kinetics-power-efficiency.php

45. What do you think?
Ideas based on
Concept Cartoons:
http://www.conceptcartoons.com
Clipart people from: http://www.clker.com/search/krug/1