Work, work, work ……

When a force
moves an object
it does work and
energy is
transferred to
the object.
 When a force
moves an object
Work, work, work …… it does work and
energy is
transferred to
the object.

Energy supplied

Work done

Energy
transferred

Amount of energy transferred (J) = Work done (J)

Work, work, work ……
When a force
moves an
object it does
work and
energy is
transferred
to the object.

The man shovelling is doing work. If he does 600J of

work, then he loses 600J of energy. The substance being
shovelled gains energy - but not the full 600J, as some is
lost as sound and heat.
 Work
• Work is done whenever a force makes
something move.
• The greater the force, and the greater
the distance moved, the more work is
done.
• When work is done energy is
transferred from one form into
another.
 Work
Work done = force x distance

W = F x d

Work is measured in Joules

 Work
Work done = force x distance

Eg. if a 4 N force moves a distance of 3m

W = 4 x 3 = 12 J
 Work
Work W=F x d

Force
W F= W
d

F d d= W
F
Distance
Types of energy…… Energy in a moving
object, eg. A
moving car

Energy contained Kinetic Energy due to the

in any hot or warm height of an
object, eg. Heat GPE object, eg. A
Burning coal skier on a slope

Energy given out Energy in a

by vibrating Elastic stretched or
Sound
objects, eg. A PE compressed object,
loud speaker
Types of eg. a spring
energy

Energy given out Energy contained

by any hot object, Light Chemical in food or fuel,
eg. The Sun, light eg. Food and
bulb. petrol

Nuclear Electrical

Energy released Energy in the flow

when the nucleus of electrons, eg.
of an atom splits A battery
Types of energy……

Gravitational
Potential Energy

Kinetic energy
 Potential Energy
• Potential energy is the energy due to height.

• Potential energy (PE) = mass x g x height

• (g is the gravitational constant = 10 m/s2)

 Potential Energy
• Eg. A sheep of mass 47kg is slowly raised
through a height of 6.3m. Find the gain in
potential energy.
• PE = mgh = 47 x 10 x 6.3 = 2961J
• As an object falls, its potential energy is
changed into kinetic energy.
• Kinetic energy GAINED = Potential energy
LOST
 Kinetic Energy
• Kinetic energy is the energy of movement.

• Anything moving has kinetic energy.

• Kinetic energy (KE) = ½ x mass x velocity2

• Or KE = ½ x mass x velocity x velocity

 Kinetic Energy
• Eg. A car of mass 2450kg is travelling at
38m/s. Calculate its kinetic energy.

• KE = ½ x 2450 x 382 = 1,768,900J

• KE depends upon mass and speed, so a larger

object travelling at greater speed has a bigger
KE.
Types of energy……

Let’s have a
look at a more
challenging
calculation
involving GPE
and KE
Eg. A mouldy tomato of mass 140g is dropped
from a height of 1.7m. Calculate its speed as it
hits the floor.

1. PE lost = mgh = 0.14 x 10 x 1.7 = 2.38 J

2. So KE gained = PE lost = 2.38 J = ½ mv2
3. So, 2.38 = ½ x 0.14 x v2 = 0.07 x v2
4. So, 2.38 = v2
0.07
5. So, 34 = v2 ; v = 5.38 m/s
Energy transfer
Energy is transferred from cells and other sources

Anything that supplies electricity is also supplying energy.

So cells, batteries, generators, etc. all transfer energy to
the charge in the wire, which then transfers it to the
components or devices in the Mcircuit:

M
Power supply Kinetic Light Heat
provides the energy energy energy
230 V energy

Sound
energy
Energy transfer
We can also use a Sankey diagram to show energy transfer

A Sankey diagram makes it easier to see how much input

energy is being usefully employed compared with how much
is being wasted. The thicker the arrow, the more energy it
represents.

USEFUL SOUND ENERGY

ENERGY
USEFUL LIGHT ENERGY
INPUT

WASTED HEAT ENERGY

Eg. Television set

Energy transfer
We can also use a Sankey diagram to show energy transfer
 Energy transfer
We can also use a Sankey diagram to show energy transfer
Eg. Throwing a
Thermal energy
stone (wasted in body)

Thermal energy
(wasted because
of air resistance)

Chemical
Thermal energy
energy in
muscles
Sound

Kinetic Potential Kinetic Thermal energy

energy energy energy (in ground and
stone)

Stone thrown Stone at highest Stone hits the

upwards point ground
Describe the energy changes
taking place in a roller
coaster ride.
 As it travels around the
track, energy changes from
GPE to KE and back again
Total energy at any one time
= KE + GPE
When there are no resistive
forces, total energy remains
constant.
 This is known as the
principle of conservation of
energy.
Energy cannot be created or
destroyed, but only changed
from one form into another.
Usually, some energy is used
up doing work against
friction and air resistance →
lost as heat.
Power and Efficiency Examples

Total energy Engine / Useful work Efficiency

input (J) motor done (J) (%)

100 25 25
Petrol
engine

100 Diesel
engine 35 35

100 80 80
Electric
motor

100 15 15
Human
body
Power and Efficiency

Power is the rate at

which work is done.

The unit of power is

the watt (w).

One watt is energy

transferred at the
rate of one joule per
second.
Power and Efficiency

power = work done

time taken

1000 W = 1 kilowatt (kW)

Typical power outputs:
Washing machine
motor
250 W
Athlete 400 W
Small car engine 35 000 W
Large car engine 150 000 W
Large jet engine 75 000 000 W
Power and Efficiency

power = energy transformed

time taken

power = E
t
Power and Efficiency

efficiency = useful power output

total power input × 100%
Power and Efficiency Calculation examples

The weightlifter in the picture is pressing

the weight above his head 50cm each time.

a) The weightlifter spends 3 minutes doing 60 lifts of 45 kg. Work out his power output.

convert the time to seconds, = 3 x 60 = 180s

work done = force x distance = 60 x 45 x 10 x 0.5 = 13 500 J

power = work done / time taken = 13 500 / 180 = 75 W

Power and Efficiency Calculation examples

The weightlifter in the picture is pressing

the weight above his head 50cm each time.

b) Work out the weightlifter’s total power output if he does 3 sets of 10 lifts with 70kg in
5 minutes.

convert the time to seconds, = 5 x 60 = 300s

work done = force x distance = (3 x 10) x 70 x 10 x 0.5 = 10 500 J

power = work done / time taken = 10 500 / 300 = 35 W

Power and Efficiency Calculation examples

The weightlifter in the picture is pressing

the weight above his head 50cm each time.

c) Over the next 10 minutes, he does 50 lifts of 40kg, 3 sets of 10 lifts with 75kg and 2
sets of 15 lifts with 60 kg. Work out his total power output to the nearest whole
number.

convert the time to seconds, = 10 x 60 = 600s

total force = (50 x 40 x 10) + (30 x 75 x 10) + (30 x 60 x 10)

= 20 000 + 22 500 + 18 000 = 60 500

work done = 60 500 x 0.5 = 30 250 power = 30 250 / 600 = 50 W

Power and Efficiency Calculation examples

The weightlifter in the picture is pressing

the weight above his head 50cm each time.

d) The weightlifter’s maximum power output is 100 W. At maximum power, how many
times can he lift 80kg in 4 minutes?

convert the time to seconds, = 4 x 60 = 240s

power = work done / time taken 100 = (n x 80 x 10 x 0.5) / 240

100 = (n x 400) / 240 100 x 240 = n x 400

(100 x 240) / 400 = n n = 24 000 / 400 = 60 reps