Education Always

Level: JHS, Basic 8  |  Subject: Integrated Science  |  Strand: Forces and Energy  |  Topic: Energy (Work, Energy and Power)

Curriculum reference: B8.4.1.1 Demonstrate understanding of work, power and energy calculations. Indicator B8.4.1.1.1: Calculate work done, power and energy transformations. All the exemplars are merged into one set of notes.

1. Work

In science, work is done when a force moves an object in the direction of the force.

Work done = Force x Distance moved in the direction of the force

W = F x d

• Force (F) is measured in newtons (N).
• Distance (d) is measured in metres (m).
• Work (W) is measured in joules (J). One joule is the work done when a force of 1 N moves an object through 1 m.

If you push a wall and it does not move, no work is done, because the distance moved is zero.

2. Energy

Energy is the ability to do work. It is also measured in joules (J). Forms of energy include kinetic, potential, heat, light, sound, electrical, chemical and nuclear energy.

• Kinetic energy is the energy a moving object has. KE = 1/2 x m x v x v, where m is mass in kg and v is speed in m/s.
• Potential energy is the energy an object has because of its position or condition. Gravitational PE = m x g x h, where g is about 10 N/kg (or m/s squared) and h is the height in metres.

3. Energy Transformations

Energy cannot be created or destroyed. It changes from one form to another.

Device	Energy change
Electric bulb	Electrical to light and heat
Electric fan	Electrical to kinetic
Radio	Electrical to sound
Solar panel	Light to electrical
Charcoal stove	Chemical to heat and light
Falling mango	Potential to kinetic
Dry cell torch	Chemical to electrical to light

4. Power

Power is the rate of doing work, or the work done per second.

Power = Work done / Time taken

P = W / t

• Power is measured in watts (W). One watt is one joule per second.
• 1 kilowatt (kW) = 1000 W.
• Time (t) is in seconds (s).

5. Worked Examples

Example 1 (work). A boy pushes a loaded wheelbarrow with a force of 50 N through 12 m. Find the work done.
W = F x d = 50 x 12 = 600 J.

Example 2 (finding force). A porter does 900 J of work to move a box through 3 m. Find the force.
F = W / d = 900 / 3 = 300 N.

Example 3 (power). A girl does 600 J of work in 20 s. Find her power.
P = W / t = 600 / 20 = 30 W.

Example 4 (finding work from power). A pump of power 500 W runs for 40 s. Find the work done.
W = P x t = 500 x 40 = 20 000 J.

Example 5 (potential energy). A coconut of mass 2 kg is on a tree 5 m above the ground. Take g = 10 N/kg. PE = m x g x h = 2 x 10 x 5 = 100 J.

Example 6 (kinetic energy). A ball of mass 0.5 kg moves at 4 m/s. KE = 1/2 x 0.5 x 4 x 4 = 4 J.

6. Quick Formula Guide

Quantity	Formula	Unit
Work	W = F x d	joule (J)
Power	P = W / t	watt (W)
Force	F = W / d	newton (N)
Time	t = W / P	second (s)
Potential energy	PE = m x g x h	joule (J)
Kinetic energy	KE = 1/2 x m x v squared	joule (J)

Remember: Always write the formula, put in the numbers with their units, and give the answer with the correct unit.

Education Always (education.inspiringalways.com). Original notes written to follow the NaCCA Basic 8 Integrated Science curriculum.