What you'll learn
This revision guide covers everything you need to know about work done and energy transfer for AQA GCSE Physics. You'll learn how to calculate work done by forces, understand energy transfers between stores, and apply power calculations. These concepts appear frequently in both Foundation and Higher tier papers, particularly in calculation questions worth 3-5 marks.
Key terms and definitions
Work done — the energy transferred when a force moves an object through a distance; measured in joules (J)
Energy transfer — the process by which energy moves from one store to another or from one object to another
Power — the rate at which energy is transferred or work is done; measured in watts (W)
Joule — the unit of energy and work; one joule equals the work done when a force of one newton moves an object one metre in the direction of the force
Gravitational potential energy (GPE) — the energy stored in an object due to its position in a gravitational field
Kinetic energy — the energy stored in a moving object
Elastic potential energy — the energy stored in a stretched or compressed object
Dissipation — the spreading out of energy into less useful forms, typically thermal energy in the surroundings
Core concepts
Work done by a force
When a force causes an object to move, work is done and energy is transferred. The amount of work done depends on both the size of the force and the distance moved in the direction of the force.
The equation for work done is:
W = F × s
Where:
- W = work done in joules (J)
- F = force applied in newtons (N)
- s = distance moved in the direction of the force in metres (m)
Key points about work done:
- Work is only done when the force causes movement
- If the force and movement are in opposite directions, negative work is done (energy is removed)
- No work is done if there's no movement, even with a large force (e.g., pushing against a wall that doesn't move)
- One joule of work is done when a force of one newton moves an object one metre
Energy transfers and stores
Energy cannot be created or destroyed, only transferred from one store to another (conservation of energy). When work is done, energy is transferred between stores.
The main energy stores you need to know:
- Kinetic energy store — moving objects
- Gravitational potential energy store — objects raised above ground level
- Elastic potential energy store — stretched or compressed objects
- Chemical energy store — food, fuels, batteries
- Thermal energy store — hot objects
- Nuclear energy store — atomic nuclei
- Magnetic energy store — magnets and magnetic materials
- Electrostatic energy store — charged objects
Common energy transfer pathways in work done scenarios:
- Lifting an object: chemical energy (muscles) → gravitational potential energy
- Accelerating a car: chemical energy (fuel) → kinetic energy (+ thermal energy from friction)
- Stretching a spring: kinetic energy → elastic potential energy
- Friction on moving objects: kinetic energy → thermal energy (dissipated to surroundings)
Gravitational potential energy calculations
When an object is raised above the ground, work is done against gravity and energy is transferred to the gravitational potential energy store.
The equation is:
Ep = m × g × h
Where:
- Ep = change in gravitational potential energy in joules (J)
- m = mass in kilograms (kg)
- g = gravitational field strength in newtons per kilogram (N/kg) — on Earth, g = 9.8 N/kg (often approximated to 10 N/kg in calculations)
- h = change in height in metres (m)
This equation appears on the Physics equation sheet in your exam, but you must know how to apply it correctly.
Important considerations:
- The change in height (h) is what matters, not the absolute height
- Energy is transferred to the GPE store when lifted up, from the GPE store when lowered
- This equation assumes the gravitational field strength is constant (valid near Earth's surface)
Kinetic energy calculations
Moving objects have energy in their kinetic energy store. The faster an object moves or the greater its mass, the more kinetic energy it has.
The equation is:
Ek = ½ × m × v²
Where:
- Ek = kinetic energy in joules (J)
- m = mass in kilograms (kg)
- v = speed in metres per second (m/s)
This equation is also on your formula sheet.
Key points:
- Kinetic energy is proportional to mass (double the mass, double the kinetic energy)
- Kinetic energy is proportional to the square of speed (double the speed, quadruple the kinetic energy)
- When a moving object does work against friction, its kinetic energy decreases
- This energy is transferred mainly to thermal energy stores
Elastic potential energy
When you stretch, compress, or bend an elastic object, you do work on it and energy is transferred to its elastic potential energy store.
For elastic deformation (where the object returns to its original shape):
Ee = ½ × k × e²
Where:
- Ee = elastic potential energy in joules (J)
- k = spring constant in newtons per metre (N/m)
- e = extension in metres (m)
This equation only applies when the elastic limit has not been exceeded. Beyond the elastic limit, the object is permanently deformed and the relationship no longer holds.
Applications include:
- Springs in vehicle suspension systems
- Elastic bands
- Bungee cords
- Bows used in archery
Power calculations
Power measures how quickly work is done or how quickly energy is transferred. A more powerful device transfers the same amount of energy in less time.
The power equation is:
P = E / t or P = W / t
Where:
- P = power in watts (W)
- E = energy transferred in joules (J)
- W = work done in joules (J)
- t = time in seconds (s)
Alternative form using work done:
P = F × v
Where:
- P = power in watts (W)
- F = force in newtons (N)
- v = speed in metres per second (m/s)
This form is useful when an object moves at constant speed with a constant force.
One watt equals one joule per second (1 W = 1 J/s).
Common power values for context:
- LED light bulb: 5-10 W
- Laptop computer: 50-100 W
- Microwave oven: 700-1000 W
- Electric car motor: 50,000-200,000 W (50-200 kW)
Worked examples
Example 1: Calculating work done (Foundation/Higher)
Question: A student pulls a suitcase with a force of 45 N across an airport terminal. The suitcase moves 120 m in the direction of the force. Calculate the work done by the student. (3 marks)
Solution:
Step 1: Write down the equation W = F × s (1 mark)
Step 2: Substitute values W = 45 × 120 (1 mark)
Step 3: Calculate and state unit W = 5400 J (1 mark)
Mark scheme notes: You must show your working to gain method marks even if your final answer is wrong. Always include the unit for full marks.
Example 2: Energy transfer involving GPE and KE (Higher)
Question: A roller coaster car of mass 500 kg is at the top of a hill 25 m high. It rolls down the hill and reaches the bottom travelling at 18 m/s. Calculate: (a) The gravitational potential energy at the top of the hill (2 marks) (b) The kinetic energy at the bottom of the hill (2 marks) (c) Explain why these values are different (2 marks)
Solution:
(a) Ep = m × g × h Ep = 500 × 10 × 25 (or 500 × 9.8 × 25) Ep = 125,000 J (or 122,500 J) (2 marks)
(b) Ek = ½ × m × v² Ek = 0.5 × 500 × 18² Ek = 0.5 × 500 × 324 Ek = 81,000 J (2 marks)
(c) The kinetic energy is less than the initial gravitational potential energy (1 mark) because some energy has been dissipated/transferred to thermal energy stores due to friction/air resistance (1 mark).
Example 3: Power calculation (Foundation/Higher)
Question: An electric motor lifts a 200 kg load through a height of 15 m in 30 seconds. The gravitational field strength is 10 N/kg. (a) Calculate the work done by the motor (3 marks) (b) Calculate the power output of the motor (2 marks)
Solution:
(a) Work done = force × distance Force needed = weight = m × g = 200 × 10 = 2000 N (1 mark) W = F × s = 2000 × 15 (1 mark) W = 30,000 J (1 mark)
Alternative method using GPE: Ep = m × g × h = 200 × 10 × 15 = 30,000 J (full marks)
(b) P = W / t (or P = E / t) P = 30,000 / 30 (1 mark) P = 1000 W (or 1 kW) (1 mark)
Common mistakes and how to avoid them
Confusing distance with displacement or height — Always use the distance moved in the direction of the force. For vertical motion, use the change in height, not the total distance travelled.
Forgetting to square the velocity in kinetic energy calculations — The equation is ½mv², not ½m × v. Calculate v² separately to avoid errors (e.g., if v = 20 m/s, then v² = 400 m²/s²).
Mixing up units — Work, energy, and all energy stores are measured in joules (J). Power is measured in watts (W). Convert all measurements to standard units: mass in kg, distance in m, time in s, force in N.
Using the wrong value for g — In exam questions, use g = 9.8 N/kg unless told otherwise. Many questions state "g = 10 N/kg" to simplify calculations. Always check what value to use.
Ignoring energy dissipation — Real-world scenarios involve friction and air resistance. When calculating energy transfers, remember that not all energy transfers to the useful store — some is always dissipated to thermal energy stores.
Incorrect rearrangement of equations — Practice rearranging the work done, power, GPE, and KE equations. Use the triangle method or algebraic rearrangement, and check your answer makes sense (e.g., higher speed should give higher kinetic energy).
Exam technique for "Work done and energy transfer"
Identify command words precisely — "Calculate" requires numerical working and an answer with units (usually 2-4 marks). "Describe" needs a written explanation without calculations (2-3 marks). "Explain" requires reasoning, often linking cause and effect (2-4 marks).
Show all working in calculations — Even if you make an arithmetic error, you can still earn method marks for using the correct equation and substitution. Write the equation, substitute values, then calculate.
Check energy conservation — In multi-step problems, total energy in = total energy out (plus any dissipated energy). Use this principle to check your answers or find missing values.
Use appropriate precision — Give answers to 2 or 3 significant figures unless told otherwise. For g = 10 N/kg calculations, 2 significant figures is usually sufficient. Don't round intermediate calculations.
Quick revision summary
Work is done when a force moves an object (W = F × s). Energy transfers between stores when work is done — lifting increases GPE (Ep = mgh), accelerating increases KE (Ek = ½mv²), and stretching increases elastic PE (Ee = ½ke²). Power measures the rate of energy transfer (P = E/t). Remember that energy is always conserved but often dissipated to thermal stores through friction. All energy and work values are in joules; power is in watts. Practice rearranging equations and always show your working in exam calculations.