Work done and energy transfer — AQA GCSE Physics
Test yourself on Work done and energy transfer with AQA GCSE practice questions.
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Work done and energy transfer explained
Work is the name physics gives to energy transferred by a force.
Read the full explanation
A force on its own transfers nothing: a person holding a heavy box still, or pushing on a wall that will not move, does no work in the physics sense however tired they feel. The object has to move, and only the part of the movement along the line in which the force acts counts, so carrying a box horizontally at a steady height does no work against its weight. When work is done the energy comes out of one store and goes into another - chemical in a muscle or an engine into kinetic as a car speeds up, or into gravitational potential as a crate is lifted. Work done and energy transferred are the same quantity measured in joules, which is why the two phrases are interchangeable in mark schemes.
So a force does work on an object when the force causes a displacement of the object.
The word displacement is doing real work in this sentence: what counts is how far the object moves in the direction the force acts, not how far it travels altogether. A porter walking along a level corridor with a suitcase exerts an upward force while the case moves sideways, so force and displacement are at right angles and no work is done against the weight. A normal contact force transfers no energy when motion is perpendicular to it, but it does work if the surface moves in the direction of the force, like an ascending lift floor. Where force and displacement point the same way, energy flows into the object; where they point opposite ways, as with friction on a sliding box, energy flows out of its kinetic store instead.
The work done by a force on an object can be calculated using the equation: work done = force × distance moved along the line of action of the force W = F s work done, W, in joules, J force, F, in newtons, N distance, s, in metres One joule of work is done when a force of one newton causes a displacement of one metre.
The equation says that the joules transferred are the newtons multiplied by the metres moved along the line of action of the force. Both quantities must be in standard SI units: a force in kilonewtons or a distance in centimetres has to be converted first, and questions put one of them in the wrong unit on purpose. A 250 N push that moves a trolley 8.0 m does 250 × 8.0 = 2000 J of work. Rearranged, F = W/s and s = W/F, and mark schemes credit any correct rearrangement, so the safest route is to write the equation, substitute, then rearrange. The definition sits behind the unit: one joule is transferred when one newton moves its point of application one metre, which is why the joule and the newton-metre are the same size.
1 joule = 1 newton-metre Students should be able to describe the energy transfer involved when work is done.
One joule and one newton-metre are the same amount of energy, because multiplying newtons by metres is exactly what the work equation does. Describing a transfer means naming both ends of it. Lifting a box: the chemical store in the muscles empties into the gravitational potential store of the box. Braking a car: the kinetic store of the car empties into the thermal store of the brakes and the surroundings. The pathway here is mechanical working (work done against friction), not heating. Stretching a spring: work done by the pulling force fills the elastic potential store. Examiners are strict here - friction on its own earns nothing, and neither does 'energy is lost'. The answer has to say work is done, name the stores, and where a temperature rises say that the internal energy has increased.
Students should be able to convert between newton-metres and joules.
This is the easiest conversion in the specification, because the numbers do not change: one newton-metre is one joule, so 35 N m of work is 35 J, and 6.0 kJ is 6000 N m. What the question really tests is whether you can see that force multiplied by distance produces the joule, so a value already given in newton-metres is an energy and needs no factor at all. The care is needed elsewhere. A moment is also measured in newton-metres but it is a turning effect, not an energy, so it is never rewritten in joules. And if a work done comes out in kilojoules or megajoules, convert it to joules by multiplying by a thousand or by a million before using the number in a later equation such as the kinetic energy one.
Work done against the frictional forces acting on an object causes a rise in the temperature of the object.
Friction opposes motion, so anything sliding has work done against it, and that work does not vanish - it is transferred to the internal energy store of the two surfaces and they get hotter. Inside a car brake the pads rub the disc, the kinetic store of the car empties, and the internal energy of the brakes rises until they are hot enough to transfer energy to the surrounding air by heating. The same mechanism warms a rope pulled through the hands and a drill bit in metal. At particle level the surfaces are rough, so rubbing makes their particles vibrate faster, and that is what a higher temperature means. Because the energy then spreads into the surroundings it is usually called wasted, although none of it has been destroyed.
Your focus
- Decide whether work is done in a given situation by testing whether the force moves its point of application.
- Name the store energy leaves and the store it enters for a lift, a push and a braking car.
- Justify why holding a mass stationary transfers no energy by work, even though the muscle feels the effort.
Show all 18 objectives
- Explain why a force acting at right angles to the motion transfers no energy, using a load carried along a level floor.
- Compare the energy transfer when a force acts with the motion and when it opposes the motion.
- Predict whether the kinetic store of an object rises or falls from the directions of the force and the displacement.
- Recall the work equation and rearrange it to make force or distance the subject.
- Calculate the work done when a 250 N force moves a trolley 8.0 m, giving the answer in joules.
- Convert a force in kilonewtons and a distance in centimetres before substituting them.
- State the equivalence of the joule and the newton-metre using the work equation.
- Describe the energy transfer for a lifted box, a braking car and a stretched spring, naming both stores.
- Explain why an answer of friction alone gains no credit for the temperature rise in a set of brakes, and identify mechanical working as the pathway.
- Convert a work done of 35 N m into joules and 6.0 kJ into newton-metres.
- Distinguish a moment in newton-metres from an energy in newton-metres by what each quantity measures.
- Convert kilojoules into joules before substituting into a kinetic energy calculation.
- Explain the temperature rise of a car's brakes in terms of work done against friction.
- Describe at particle level why rubbing two surfaces together raises their temperature.
- Identify which energy store empties and which fills as an object slides to a stop.
Work done and energy transfer exam tips
Quick Revision Summary (Key Takeaway)
Work done is a measure of mechanical or electrical energy transfer that occurs whenever a force moves an object through a distance along the line of action. It is calculated using W = Fs, where one joule of work is done when a force of one newton displaces an object by one metre.
Topic Overview
Work done and energy transfer forms the mechanical foundation of AQA GCSE Physics Topic 1 (Energy). Whenever an object is moved by an applied force, energy is mechanically shifted out of one store and into another, meaning that 'work done' is simply another phrase for energy transferred.
Mastering this topic is essential because it bridges kinematics and dynamics with conservation of energy, power calculations, and mechanical efficiency across both Paper 1 and Paper 2. It equips students to evaluate braking systems, electric motors, and everyday forces in structured multi-step calculations.
Key Concepts
- →Work is done when a force causes an object to be displaced through a distance along the line of action of that force (W = Fs).
- →One joule of work is done when a force of one newton causes a displacement of one metre (1 J = 1 N m).
- →Work done against frictional forces acting on an object causes a rise in the temperature of the object and its surroundings, dissipating energy into thermal stores.
- →Power is the rate at which work is done or energy is transferred (P = W / t = E / t), measured in watts (W).
Marking Points
- one mark for stating that the force must move the object through a distance before any work is done
- one mark for using the distance moved along the line of action of the force, not the whole path
- one mark for naming the energy store the transfer comes from and the store it fills
- one mark for treating work done and energy transferred as the same quantity, measured in joules
- one mark for linking the force to a displacement in the direction of that force
- one mark for stating that a force at right angles to the motion does no work
- one mark for energy transferred to the object when force and displacement are in the same direction
- one mark for a force opposing the motion removing energy from the object
- one mark for the equation work done = force × distance, or any correct rearrangement
- one mark for substituting values converted to newtons and metres
- one mark for the correctly calculated work done
- one mark for the unit J, or N m, given with the answer
- one mark for stating that one joule is the work done when one newton moves through one metre
- one mark for naming the store the energy is transferred from
- one mark for naming the store the energy is transferred to, and the pathway, such as mechanical working against friction
- one mark for describing a temperature rise as an increase in the internal energy of the object
- one mark for using one newton-metre as equal to one joule, with no numerical change
- one mark for converting kilojoules or megajoules into joules before substituting
- one mark for leaving a moment in newton-metres rather than converting it to joules
- one mark for the quantity quoted with the unit the question asks for
- one mark for stating that work is done against friction between the surfaces
- one mark for the kinetic energy store of the moving object decreasing
- one mark for an increase in the internal, or thermal, energy store, so the temperature rises
- one mark for the energy then being transferred to the surroundings by heating
Examiner Tips
- 💡Check the object actually moves before writing that work is done; a stationary object means the answer is zero.
- 💡Name the store that empties and the store that fills - examiners want the transfer named, not just the word energy.
- 💡Draw the force arrow and the movement arrow; if they are perpendicular the work is zero.
- 💡If the force opposes the motion, say that the kinetic store falls - that link is usually the mark.
- 💡Write the equation before substituting; the equation is itself a mark and it survives an arithmetic slip.
- 💡Check the units before you multiply - kilonewtons and centimetres are the usual traps.
- 💡Build the answer as one sentence with two stores in it: from this store to that store.
- 💡If something gets warmer, say the internal energy increases; heat energy is not a store.
- 💡If the unit is N m and the quantity is an energy, simply change the label to J.
- 💡Check whether the newton-metre in front of you is a moment or a work done before doing anything with it.
- 💡Use the phrase work is done against friction; it is the marking point examiners look for.
- 💡Name internal energy, not heat, as the store that fills.
- 💡State the base formula (e.g. W = Fs) explicitly before substituting values, as this secures method marks even if a calculation slips.
- 💡Always check whether the answer requires kilojoules (kJ) or joules (J), and round to the lowest number of significant figures provided in the question data.
- 💡When explaining friction, always use the phrase 'energy is transferred to thermal stores of the surroundings' rather than vague words like 'lost' or 'used up'.
Common Mistakes
- saying work is done while someone holds a heavy load still, when the load does not move
- using the full path length where the force acts at right angles to part of the motion, such as the weight of a bag carried along a level floor
- writing that the force makes energy rather than transferring it between stores
- confusing work done with power by leaving a time in the answer
- treating any movement as work, including motion at right angles to the force such as a load carried on the level
- saying the normal contact force from the ground does work on a walker moving horizontally
- ignoring that friction acts in the opposite sense and so empties the kinetic store
- using the length of a curved path rather than the displacement in the direction of the force
- leaving a force in kilonewtons or a distance in centimetres in the substitution
- multiplying by the whole journey length when part of it is at right angles to the force
- rearranging the equation to work done = force divided by distance
- quoting the answer in newtons because the force was the larger number in the question
- answering friction alone, which mark schemes explicitly ignore; instead, state that work is done against friction
- writing that energy is lost instead of naming the store it was transferred to
- treating the newton-metre as a different unit from the joule and converting with a factor; they are equivalent
- naming heat as a store rather than internal energy
- incorrectly identifying the pathway for friction as heating rather than mechanical working
- multiplying or dividing by a conversion factor between newton-metres and joules
- converting the moment of a force into joules because the unit looks the same
- leaving an energy in kilojoules when the next equation needs joules
- writing the unit as newtons per metre instead of newton-metres
- writing friction alone, which the mark scheme ignores without the work done
- saying the energy is destroyed or lost rather than transferred to an internal energy store
- claiming the brakes get hot because of air resistance acting on the car
- confusing temperature with internal energy when saying what has risen
- Thinking that holding a stationary heavy object requires work to be done on the object; because displacement is zero (s = 0), mechanical work done on the object is exactly 0 J.
- Believing that work done and energy are different physical quantities with different units; both are scalar quantities measured in joules (J).
- Assuming normal contact forces do work during horizontal motion; forces perpendicular to displacement do zero mechanical work.
Revision Plan
- 1Day 1-2: Memorise the formula W = Fs, practice standard unit conversions (cm/km to m; kJ to J), and learn the standard definition of the joule.
- 2Day 3-4: Work through combined multi-step problems that link weight (W = mg) or braking force with kinetic energy (Ek = 0.5mv^2).
- 3Day 5-6: Practice descriptive 4-6 mark questions explaining energy store transfers during friction, braking, and vertical lifting.
- 4Day 7: Complete timed past paper calculation questions from AQA Paper 1, strictly self-marking using official mark schemes.
Exam Question Types
- 📋Direct substitution calculation questions (2-3 marks): Calculate work done, force, or distance using W = Fs with unit conversions.
- 📋Multi-step energy links (4-5 marks): Combining work done against braking forces with initial kinetic energy to calculate stopping distance.
- 📋Extended response explanations (4-6 marks): Describing thermal dissipation and energy transfers when work is done against resistive forces.
Command Word Expectations (AQA)
Show clear working: state the formula, substitute converted numbers, evaluate the mathematical answer, and supply correct units.
State the key stages of an energy transfer clearly without needing to explain why; identify the start store, pathway, and destination store.
Provide a reasoning link, using cause-and-effect language (e.g. 'work done against friction causes an increase in thermal energy store').
How Students Lose Marks (Examiner Pitfalls)
Step-by-Step Worked Solutions
Question: A cyclist applies a driving force of 140 N to travel a distance of 850 m along a flat road against resistive forces. Calculate the work done by the cyclist and state the energy transfer that occurs.
- 1.Step 1: Identify given quantities and formula. Force (F) = 140 N, distance (s) = 850 m. Equation: W = F * s.
- 2.Step 2: Substitute the numerical values into the equation: W = 140 N * 850 m = 119,000 J (or 119 kJ).
- 3.Step 3: State the energy pathway. Chemical energy stored in the cyclist is transferred mechanically to kinetic energy and thermal energy stores in the surroundings via friction and air resistance.
Question: An electric hoist raises a 65 kg builder's bag of sand vertically by 4.0 m at constant speed. Gravitational field strength g = 9.8 N/kg. Calculate the work done on the sand.
- 1.Step 1: Calculate the force required to lift the sand against gravity using W_weight = m * g. Weight = 65 kg * 9.8 N/kg = 637 N.
- 2.Step 2: Use the work done formula W = F * s where s is the vertical displacement: W = 637 N * 4.0 m.
- 3.Step 3: Calculate final numerical value: W = 2548 J. Round to 2 significant figures to match the given data: 2500 J (or 2.5 kJ).