Energy and Work
1. Introduction
Work and energy are closely related concepts in physics.
Energy is the capacity to do work.
Work is done when a force moves an object through a distance.
2. Work
In everyday language, 'work' means any activity involving effort. But in physics, work has a specific meaning.
Scientific definition: Work is done when a force applied on an object moves it through a distance in the direction of the force.
Formula for Work
Work = Force x Displacement
W = F x d
Units:
- Force in newtons (N), distance in metres (m).
- Work is measured in joules (J).
- 1 joule = 1 newton x 1 metre.
When is work NOT done?
- When you push a wall but it does not move (no displacement).
- When you carry a bag while standing still.
- When a satellite moves in a circular orbit (force is perpendicular to displacement).
Worked Example: A force of 50 N is applied to push a box 2 metres. Calculate the work done.
W = F x d = 50 N x 2 m = 100 J.
Common Mistake: Thinking work is done when you hold a heavy object stationary. No displacement means no work in the physics sense!
Exam Focus (3 marks): 'Calculate the work done in lifting a 10 kg object to a height of 2 m. (g = 10 m/s^2)'
Force = weight = m x g = 10 x 10 = 100 N.
W = F x d = 100 x 2 = 200 J.
3. Energy
Energy is defined as the capacity to do work. The more energy an object has, the more work it can do.
Unit of energy: joule (J), same as work.
Principle of Conservation of Energy:
Energy can neither be created nor destroyed. It can only be converted from one form to another.
4. Forms of Energy
| Form | Description | Examples |
|---|---|---|
| Kinetic energy (KE) | Energy of motion | Moving car, flowing water, wind |
| Potential energy (PE) | Stored energy | Water in a dam, stretched bow |
| Heat energy | Energy of moving particles | Fire, sun, friction |
| Light energy | Visible form of energy | Sun, bulb, candle |
| Sound energy | Energy of vibrations | Music, speech, bell |
| Electrical energy | Flow of electrons | Batteries, power lines |
| Chemical energy | Stored in chemical bonds | Food, fuel, battery |
| Nuclear energy | Energy in atomic nucleus | Nuclear power plants |
Worked Example: For each, identify the form of energy: (a) A running athlete (b) A battery (c) A flying bird.
(a) Kinetic energy (motion).
(b) Chemical energy (stored in chemicals).
(c) Both kinetic (motion) and potential (height above ground).
Kinetic Energy
Energy possessed by a moving object.
Depends on: mass and speed.
The faster an object moves, the more kinetic energy it has. The heavier an object, the more kinetic energy.
Potential Energy
Stored energy due to position or configuration.
- Gravitational potential energy: Energy due to height. PE = m x g x h.
- Elastic potential energy: Energy stored in stretched/compressed objects (spring, rubber band).
Common Mistake: Thinking a stationary object at a height has no energy. It has gravitational potential energy because of its position.
5. Sources of Energy
Renewable Sources (can be replenished naturally)
| Source | How it works |
|---|---|
| Solar energy | Sunlight converted to electricity/heat |
| Wind energy | Wind turns turbines to generate electricity |
| Hydel energy | Flowing water turns turbines |
| Tidal energy | Ocean tides generate power |
| Geothermal energy | Heat from inside the Earth |
Non-Renewable Sources (limited, will run out)
| Source | How it works |
|---|---|
| Coal | Burned to produce heat/electricity |
| Petroleum (oil) | Refined into fuel |
| Natural gas | Burned for heating/electricity |
| Nuclear fuel | Nuclear fission releases energy |
Exam Focus (3 marks): 'Give two reasons why we should use renewable sources of energy.'
(1) Renewable sources do not get exhausted and are available naturally.
(2) They are environment-friendly and do not produce harmful pollutants.
6. Comparison Table: Work vs Energy
| Aspect | Work | Energy |
|---|---|---|
| Definition | Force x displacement | Capacity to do work |
| Nature | Process (transfer of energy) | Property (stored in object) |
| Formula | W = F x d | Various forms |
| Unit | Joule (J) | Joule (J) |
7. Self-Test
- Define work in physics. When is work said to be done?
- Calculate work done: A force of 25 N moves an object 4 m.
- Define energy. State the law of conservation of energy.
- Name the energy form: (a) A stretched rubber band (b) A moving bus (c) Food.
- Differentiate between renewable and non-renewable sources of energy. Give two examples of each.
- Calculate the potential energy of a 5 kg object at a height of 3 m (g = 10 m/s^2).
- Why is energy required for living organisms?
8. Answers to Self-Test
- Work is done when a force moves an object in the direction of the force. W = F x d.
- W = 25 x 4 = 100 J.
- Energy is the capacity to do work. Law: Energy can neither be created nor destroyed, only converted.
- (a) Elastic potential energy (b) Kinetic energy (c) Chemical energy.
- Renewable: can be replenished naturally (solar, wind). Non-renewable: limited, will run out (coal, petroleum).
- PE = m x g x h = 5 x 10 x 3 = 150 J.
- All life processes (growth, movement, reproduction) require energy. Living organisms get energy from food.
