Force, energy, work and Power 8202.202.2

Level 2 Technical Certificate in Electrical Installation

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Force, energy, work and power

Unit 202: Electrical science

Level 2 Technical Certificate in Electrical Installation

© 2017 City and Guilds of London Institute. All rights reserved.

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SI units – Force

The SI unit of force is the

; Symbol

.

Definition of the newton: That force producing an acceleration of 1metre
per second (m/s), every second, when acting on a mass of 1 kilogram
(1kg).

Where…

F = Force, measured in newtons
m = mass, measured in kilograms
a = acceleration, defined in metres per second, per second (m/s2)

Newton (N)

F

Force = mass x acceleration

or: F = m.a (N)

Level 2 Technical Certificate in Electrical Installation

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SI units – Force

On Earth, a mass of 1 kilogram experiences a force of 9.81 newtons
(N) as a result of gravity.

The weight of a body is equal to the force acting on the mass of the
body as a result of the acceleration due to gravity. So, it would require
a force of 9.81 newtons to raise a mass of 1kg against the pull of
gravity (on Earth).

Example 1

Calculate the force required to overcome gravity and lift a 200 kg
object:

F = 200 x 9.81 = 1.962 kN

Math Response

What is the force acting on a 75kg weight on Mars, where the acceleration due to gravity is approximately 3.7 m/s²? (Answer in Newtons)

Type answer here

Correct Answer: 277.5

Deg^°

Rad

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SI units – Force

Now complete Worksheet 2; Questions 1 – 2

Math Response

On Earth, a mass of 1 kilogram experiences a force of 9.81 newtons as a result of gravity.

 

Q1.      Calculate the force required to overcome Earth’s gravity and lift a 125 kg object: (Answer in kN).

             

Type answer here

Correct Answer: 1.22625

Deg^°

Rad

Math Response

Calculate the mass of an object that requires a force of 7357.5 N to lift on Earth: Ans in KG

             

Type answer here

Correct Answer: 750

Deg^°

Rad

Level 2 Technical Certificate in Electrical Installation

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SI units – Energy

Energy is the capacity for doing work.

It is measured in joules (J).

Energy cannot be created or destroyed. It can only be transformed from
one form into another.

Forms of energy are…

•atomic

•electrical

•chemical

•thermal

•mechanical

Level 2 Technical Certificate in Electrical Installation

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SI units – Energy

Mechanical energy takes two forms:

Potential energy: That is; stored energy not currently being expended.

Where…

PE = Potential energy, measured in joules
m = mass, measured in kilograms
g = acceleration by gravity, defined in metres per second per second
h = height, measured in metres

Potential energy = mass x acceleration by gravity x height

or: PE = m.g.h (J)

Level 2 Technical Certificate in Electrical Installation

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SI units – Energy

Mechanical energy takes two forms:

Kinetic energy: That is; energy given out due to movement.

Where…

KE = Kinetic Energy, measured in joules
m = mass, measured in kilograms
v = velocity, defined in metres per second

Kinetic energy = mass x velocity²

2

or: 𝐾𝐸 =

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SI units – Work

Work is said to be done when energy is expended.

If the distance is 1m, and the force is 1N, the work done =

1 newton/metre (1 N/m)

1 N/m is termed 1 joule.

Work done = force  distance moved

or: 𝑊𝐷 = 𝐹𝑠
(J)

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SI units – Work

However, force is a function of mass  acceleration. So, for a vertical
lift, we can substitute m.a into the above expression…

Example 2

Calculate the work done when a body whose mass is 1000 kg is lifted
through a vertical distance of 4m.

𝐖𝐃 = 𝟏𝟎𝟎𝟎 × 𝟗. 𝟖𝟏 × 𝟒 = 𝟑𝟗𝟐𝟒𝟎 Nm

or 39.24kJ

Work done = mass  acceleration by gravity  distance moved

or: 𝑊𝐷 = 𝑚𝑎𝑠 (J)

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SI units – Work

Now complete Worksheet 2; Questions 3 – 6

Level 2 Technical Certificate in Electrical Installation

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SI units – Power

Power is the rate of doing work.

When a certain amount of work (expressed in joules) is done in a
certain amount of time (expressed in seconds), the rate of doing work
can be expressed in joules per second.

The rate of 1J per second is the unit of power, and is termed the watt.

Thus…

1 J/s = 1W

And…

Average power = work done

time taken

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SI Units – Power

However, we have seen that…

Work done = mass  acceleration by gravity  distance moved

So, substituting this into the expression, we have…

Example 3

Calculate the power required by a motor to raise a mass of 1000 kg to
a height of 5m in 1 minute.

Power =

𝟏𝟎𝟎𝟎×𝟗.𝟖𝟏×𝟓

𝟔𝟎
= 𝟖𝟏𝟕. 𝟓 W

Average power = mass  acceleration by gravity  distance moved

time taken

Level 2 Technical Certificate in Electrical Installation

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SI Units – Power

Now complete Worksheet 2; Questions 7 - 9

Level 2 Technical Certificate in Electrical Installation

© 2017 City and Guilds of London Institute. All rights reserved.

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Any questions?