1. TITLE:Work,
Energy
and Power
ISN PAGE 104

2. What is Force?
*FORCE IS A PUSH OR PULL
* FORCE IS MEASURED IN
NEWTON'S (N)

3. Work
Definition:
Work done by a constant force on an object is given by
the product of the force and the distance moved by the
object in the direction of the force. (TB)
F F
d
Work = Force x distance moved
W=Fxd

4. NO WORK DONE IF
1. the body does not move.
2. the force is perpendicular to the direction of
movement.

5. Work
Work done by force = 0

6. Work
Work is done by a force.
Example 1: Motion in different directions

7. Powerful cranes

8. Power
• *Power is the rate of work done or
rate of energy conversion.
• Formula(Write both formulas)
P= W P=E
t t P = Fv
• The SI unit = watt (W)
•1 W = 1 J/s

9. REVIEW

10. What unit is Force measured in?

11. What is measured in watts?

12. What is the equation for Work?

13. Principle of Conservation of
Energy
• It states that energy cannot be created or
destroyed in any process.
• It can be converted from one form to another or
transferred from one body to another, but the
total amount remains constant. (TB)

14. Worked Example 4
A bricklayer lifts 50 bricks each weighing 15 N
through a vertical height of 1.2 m in 1 min and
places them at rest on a wall. Calculate
(a) the work done
(b) the average power needed.

15. Energy
• Energy is the capacity to do work.
• The SI unit for energy is joule (J).
• Different forms of Energy
Kinetic Potential Heat Sound Electrical
Energy Energy Energy Energy Energy
Nuclear Chemical
Gravitational Elastic
Energy Energy

16. Energy Transformation
1. Fossil fuel is used to generate electricity in a
power station.
burn Turbine Generator
Chemical Heat Kinetic Electrical
energy in fuel energy energy of turbine energy
2. Stored water in a dam is used to generate electricity
in a hydroelectric power station.
Gravitational Kinetic Kinetic Electrical
Potential energy of water energy of turbine
energy of water
energy

17. Gravitational Potential Energy
(GPE)
• GPE is the energy possessed by a body due to
its height above the ground.
• Formula:
G.P.E. = mgh (from conversion of WD to GPE)
• where m= mass of body (in kg)
g = gravitational field strength (in N/kg)
• h = vertical height above ground (in m)

18. Factors affecting GPE

19. Worked Example 1
A brick of mass 2 kg is at a height of 3 m
above the ground. What is the gravitational
potential energy of the brick?
[Take g as 10 N/kg ]

20. Kinetic Energy (KE)
• Kinetic energy is the energy possessed by a
body due to its motion.
• Formula:
K.E. = ½ m v2
where m = mass (in kg)
v = speed (in m/s)

21. Rearranging the KE equation
What is the rearranged version of this equation for
calculating speed?
√
v = 2KE
m

22. Worked Example 2
A bullet of mass 200 g is travelling at a
speed of 60 m/s. What is the kinetic energy
of the bullet?

23. Dangerous speeding?
Use the KE = ½mv2 equation to fill in the kinetic energy
values in the table below for two cars each travelling at
two different speeds.
1,000 kg 2,000 kg
20 mph KE = 40 kJ KE = 80 kJ
40 mph KE = 160 kJ KE = 320 kJ
What factor – mass or speed – has the greatest effect on
the kinetic energy of a moving object?

24. Too much kinetic energy
Doubling the mass of a moving object doubles its kinetic
energy, but doubling the speed quadruples its kinetic
energy.
If the speed of a car is slightly above the speed limit, its
kinetic energy is much greater than it would be at the
speed limit. This means that:
 It is more difficult to stop
the car and there is more
chance of an accident.

25. GPE and KE
When a body falls from a height, its GPE is changed into
KE during the fall.
GPE
As the body gains speed, more and more
of its GPE is changed into KE.
Just before the body strikes the ground, all GPE+
its GPE is changed into KE. K.E.
The total amount of energy at the
beginning and the end is the same.
K.E.

26. Worked Example 3
A ball of mass 2 kg is released from rest at the top of a
building of height 12 m.
(a) What is the speed of the ball when it reaches the
ground?
(b) What happens to its kinetic energy after it has struck
the ground?

27. Worked Example 5
Solution
(a) By the principle of conservation of energy,
K.E. = GPE
½ m v2 = mgh
v = 2gh = 2x10x12
= 15.5 m/s
(b) The kinetic energy is changed into elastic PE and heat
and sound.

28. Note
• Whenever there is friction or impact:
• heat (mostly) and sound are produced.