Work, Energy and Power – JSS2 Intermediate Science Lesson Note (Third Term)

Work is done when a force moves an object over a distance. Energy is the ability to do work. Power is the rate of doing work.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Define work, energy and power.
2. State the units of work, energy and power.
3. Calculate work done and power using their formulae.
4. Distinguish between potential energy and kinetic energy, with examples.
5. Explain how work, energy and power are related.

What is work in science?

Work is done when a force moves an object over a distance in the direction of the force.

• Everyday meaning: we say we are “working” when we sit at a desk or study for an exam.
• Scientific meaning: work needs a force and movement. Without movement, no work is done.
• Force: a push or a pull. Its unit is the newton (N).
• Distance: how far the object moves, measured in metres (m).

What two conditions must be met for work to be done?

1. A force must be applied.
2. The object must move in the direction of that force.

When is no work done?

• Pushing a wall: you push with all your strength and sweat, but the wall does not move. Work done is zero.
• Pushing a stationary truck: the truck stays in place, so no work is done, even after one hour.
• Holding a bag still: you apply a force upwards, but the bag does not move. No work is done.

How do we calculate work done?

Work done = Force × Distance.

• Symbols: W = F × d.
• Unit: the joule (J).
• One joule: the work done when a force of 1 N moves an object through 1 m.

Worked example 1:

1. A force of 10 N pushes a chair 2 m across the room.
2. Work done = 10 × 2.
3. Work done = 20 J.

Worked example 2:

1. A girl pulls a box with a force of 50 N for 4 m.
2. Work done = 50 × 4.
3. Work done = 200 J.

What is energy?

Energy is the ability to do work.

• Link to work: work is the action, and energy allows the action to happen. You cannot push a chair if you are too tired to move.
• Unit: the joule (J), the same as work, because work is energy being transferred.
• Source for people: the food we eat.
• Source for a car: petrol.

What are the two types of mechanical energy?

There are two types of mechanical energy: potential energy and kinetic energy.

What is potential energy?

Potential energy is the stored energy an object has because of its position or condition.

• Stone on top of a hill: it can fall and do work because of its height.
• Stretched catapult: the stretched rubber stores energy ready to launch a stone.
• Water in a tank on a tower: it can flow down because of its high position.

What is kinetic energy?

Kinetic energy is the energy an object has because it is moving.

• Rolling ball: it can knock down objects in its path.
• Running boy: his body is in motion.
• Falling orange: its stored energy changes to motion energy as it drops.

What is the difference between potential and kinetic energy?

What are other forms of energy?

Energy exists in many forms besides mechanical energy.

• Heat energy: from a fire or the Sun.
• Light energy: from the Sun or a bulb.
• Sound energy: from a drum or a radio.
• Chemical energy: stored in food, petrol and batteries.
• Electrical energy: from NEPA/DisCo supply, generators and batteries.

How does energy change from one form to another?

Energy cannot be created or destroyed. It can only change from one form to another.

• Electric fan: electrical energy changes to kinetic energy.
• Torch: chemical energy in the battery changes to light energy.
• Burning firewood: chemical energy changes to heat and light energy.
• Solar panel: light energy changes to electrical energy.
• Eating food: chemical energy in food changes to kinetic energy when you move.

What is power?

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

• Musa and Chidi: both carry a heavy bag of cement up a flight of stairs, so both do the same work.
• Musa: he runs up in 10 seconds.
• Chidi: he takes 2 minutes (120 seconds).
• Result: Musa has more power because he did the same work in less time.

How do we calculate power?

Power = Work done ÷ Time taken.

• Symbols: P = W ÷ t.
• Unit: the watt (W).
• One watt: one joule of work done every second (1 W = 1 J/s).
• Larger unit: 1 kilowatt (kW) = 1000 W.

Worked example:

1. A machine does 600 J of work in 20 s.
2. Power = 600 ÷ 20.
3. Power = 30 W.

Real-life example: a bulb labelled “60 W” uses 60 joules of energy every second to give light.

How are work, energy and power related?

Energy is what you have, work is what you do with it, and power is how fast you do it.

1. Energy is the “money” in your pocket (what you have).
2. Work is the “spending” (what you do with the money).
3. Power is how “fast” you spend it.

What is the chain of events?

1. You eat food and get energy.
2. You use that energy to push a box, so you do work.
3. How quickly you push the box shows your power.

What is the summary of work, energy and power?

Evaluation

1. A man pushes a stationary truck for one hour, but the truck does not move. Has he done any work in the scientific sense? Why?
2. What is the difference between potential and kinetic energy?
3. If a machine does 100 J of work in 2 seconds, what is its power? (Hint: Work ÷ Time.)
4. Why do work and energy use the same unit (joules)?
5. A boy pushes a cart with a force of 30 N for 5 m. Calculate the work done.

Assignment

Write the definitions and units of work, energy and power. Then give one example of energy changing from one form to another in your home.