WorkCambridge IGCSE Physics: Revision notes
Section 1
What is work in physics?
In physics, work is done when a force acts on an object and causes it to move in the direction of that force. Work is not about effort or difficulty — it is a precise measurement of energy transfer. If a force is applied but no movement occurs in the direction of the force, then no work is done, regardless of how hard you push. For example, holding a heavy book stationary does no work because there is no displacement in the direction of the applied force.
Work is measured in joules (J), the same unit as energy. This is because work and energy are directly linked: work done on an object equals the energy transferred to that object.
Think of work like pushing a shopping trolley: you only do work when the trolley actually moves forward. Pushing against a wall does no work because nothing moves.
Section 2
How do we calculate work done?
Work is calculated using the formula:
W = Fd
Where:
- W = work done (in joules, J)
- F = force applied (in newtons, N)
- d = displacement (in metres, m) in the direction of the force
This formula applies when the force acts in the direction of motion. Work is directly proportional to both the force applied and the distance moved — if you double the force or double the distance, you double the work done.
Important: The displacement must be measured in the direction of the force. If a force is applied at an angle to the direction of motion, only the component of force in the direction of motion counts (though this advanced calculation is not required for IGCSE).
A person pushes a lawnmower with a force of 50 N across a lawn for a distance of 20 m. Work done = F × d = 50 × 20 = 1000 J. The lawnmower has 1000 joules of energy transferred to it.
Always check units: force must be in newtons and displacement in metres to get work in joules. Examiners often test whether you can correctly apply the formula with the right units.
Section 3
What is the relationship between work and energy transfer?
Work done equals the energy transferred. This is expressed as:
W = ΔE
Where:
- W = work done (in joules, J)
- ΔE = change in energy (in joules, J)
This equation is fundamental to understanding energy in physics. When work is done on an object, energy is transferred to it. Conversely, when an object does work on something else, it loses energy. The amount of energy transferred always equals the amount of work done.
Examples of energy transfer through work:
- A person lifting a box does work against gravity and increases the gravitational potential energy of the box
- A moving car's brakes do work on the car, removing kinetic energy and transferring it as heat
- An electric motor does work, converting electrical energy into mechanical energy
- A person pulling a sled does work, increasing the kinetic energy of the sled
Work and energy transfer are two ways of describing the same thing. Work is the 'process' of energy moving; energy transfer is the 'result'. A crane lifting a steel beam does work, and the beam gains potential energy — these are the same event described differently.
Students often think work and energy are different things. Remember: work is the mechanism by which energy is transferred. They are linked by the equation W = ΔE.
Section 4
How does mechanical work relate to everyday situations?
Mechanical work occurs whenever a force moves an object through a distance. This covers most everyday scenarios:
| Situation | Force | Displacement | Work Done |
|---|---|---|---|
| Lifting a weight | Weight (downward) | Upward motion | Yes — increases potential energy |
| Dragging a box | Applied force | Motion in direction of force | Yes — overcomes friction |
| Climbing stairs | Leg muscles | Upward motion | Yes — increases potential energy |
| Pushing against a wall | Applied force | No displacement | No — object does not move |
| Dropping an object | Gravity | Downward motion | Yes — gravitational potential energy converts to kinetic |
In all situations where work is done, energy is transferred. The formula W = Fd allows you to calculate how much energy has been transferred in any mechanical process. Understanding this link is essential for solving problems about energy conservation and transformation.
A student of mass 50 kg climbs a staircase of vertical height 10 m. The gravitational force is F = mg = 50 × 10 = 500 N. Work done = F × d = 500 × 10 = 5000 J. This equals the increase in gravitational potential energy gained.
Section 5
How does electrical work relate to energy transfer?
Electrical work is done whenever an electrical device operates, converting electrical energy into other forms. Although the detailed calculations for electrical work are studied in greater depth in later topics, it is important to understand that the same principle applies:
W = ΔE — the work done equals the energy transferred
When electrical current flows through a device:
- A light bulb converts electrical energy into light and heat energy
- An electric motor converts electrical energy into mechanical energy (and some heat)
- A kettle converts electrical energy into thermal (heat) energy
- A battery does work by transferring chemical energy into electrical energy
In each case, the electrical work done on the device equals the energy transferred from electrical form to another form. The concept remains the same as mechanical work — energy is transferred, and the amount transferred equals the work done.
Examiners may ask you to recognise that W = ΔE applies to both mechanical and electrical situations. Always state clearly that work done equals energy transferred, regardless of whether the work is mechanical or electrical.
Must Know
- Work is done only when a force causes displacement in the direction of that force. Holding a stationary object does no work, even if significant effort is required.
- W = Fd: Work done (in joules) equals force (in newtons) multiplied by displacement (in metres) when force acts in the direction of motion.
- W = ΔE: Work done always equals the change in energy transferred. This is the fundamental link between work and energy.
- Work and energy transfer are the same concept: When work is done on an object, energy is transferred to it. The amount of work done equals the amount of energy transferred.
- Work applies to both mechanical and electrical situations. Whether a force moves an object or electrical current flows through a device, the principle remains: work done = energy transferred.
- Always use consistent units: Force in newtons, displacement in metres, energy in joules. Incorrect units are a common source of errors.
That's the notes covered.
Carry on to the next subtopic.