OCR GCSE Physics A: Work done, stored energy and power from forces (J249)

This guide is aligned to the OCR GCSE (9-1) Gateway Science Physics A specification (J249), Version 5.0, July 2026, for first assessment in 2018. It covers statements P2.2m, P2.2n and P2.2o in topic P2.2 Newton’s laws: work done by a force, stored energy and energy transfers, and power. None of these three statements is marked as Higher Tier only, so all of it is for both tiers.

Other P2.2 statements (free body diagrams, Newton’s second law, momentum, circular motion) are taught on other pages. The wider energy ideas are in topic P7, so this page keeps to force-based situations: a force moves something, and energy is transferred.

Related pages: OCR GCSE Physics hub, printable checklist, revision notes for this unit, practice questions for this unit, the wider OCR GCSE Physics forces guide and its practice set. If you are not sure where your gaps are, try the free 10-minute diagnostics.

What this unit covers

Statement	What you must be able to do
P2.2m	Use the relationship between work done, force and distance moved along the line of action of the force, and describe the energy transfer involved
P2.2n	Calculate relevant values of stored energy and energy transfers; convert between newton-metres and joules
P2.2o	Explain, with reference to examples, the definition of power as the rate at which energy is transferred

The specification lists the equations work done = force × distance (along the line of action of the force) and power = work done ÷ time as ones to recall and apply. Quantities are in SI units: force in newtons (N), distance in metres (m), energy and work in joules (J), time in seconds (s) and power in watts (W).

Work done by a force (P2.2m)

A force does work when it makes something move in the direction the force acts. Work done is a transfer of energy. If a force does 100 J of work, 100 J of energy has been transferred by that force.

work done (J) = force (N) × distance moved along the line of action of the force (m)

In symbols, W = F × s. Three points matter.

• The distance is how far the object moves, measured along the same line as the force. If the force is horizontal and the object moves horizontally, use the horizontal distance.
• If there is no movement, the distance is zero, so the work done is zero. Pushing hard on a wall that does not move does no work on the wall.
• If the object moves at right angles to the force, the distance along the line of action is zero, so that force does no work.

Worked example 1: calculating work done

A shopper pushes a loaded trolley with a steady horizontal force of 85 N. The trolley moves 6.4 m in a straight line. Calculate the work done by the shopper.

W = F × s
W = 85 × 6.4
W = 544 J

The shopper transfers 544 J of energy to the trolley and its surroundings.

Worked example 2: rearranging the equation

A farmer drags a feed sack across a yard for 18 m. The farmer does 1,260 J of work. Calculate the force.

F = W ÷ s
F = 1260 ÷ 18
F = 70 N

Distance can be found the same way. A force of 36 N that does 450 J of work moves an object s = W ÷ F = 450 ÷ 36 = 12.5 m.

A formula triangle helps if you like it: W on top, F and s underneath. Always write the rearranged form first, then substitute. That keeps the working clear.

Worked example 3: when the force does no work

A porter carries a suitcase of weight 120 N along a level corridor for 30 m at constant height. The carrying force is upwards (to balance the weight) and the movement is horizontal. The movement is perpendicular to the force, so the distance along that line is 0 m.

W = 120 × 0 = 0 J

The upward force does no work on the suitcase. The porter still gets tired, but that is because muscles use energy to hold the load, not because work is done on the case.

The energy transfer involved (P2.2m)

You must describe where the energy goes. The answer depends on what the work does.

Situation	What the work does	Energy transferred to
Lifting an object at steady speed	Works against the weight	Gravitational potential energy store of the object
Speeding an object up	Increases the speed	Kinetic energy store of the object
Pulling or pushing against friction at constant speed	Works against friction	Thermal energy stores of the object and the surface (they warm up)
Braking a moving vehicle	Friction force acts over a distance	Kinetic energy store is reduced; thermal energy stores of the brakes and surroundings increase

Energy is transferred by the force doing work, and the total amount is conserved. The work done equals the energy transferred.

Worked example 4: work against friction

A sledge is pulled at constant speed along a flat, rough track for 25 m. The pulling force is 18 N.

W = F × s = 18 × 25 = 450 J

The sledge is not speeding up, so its kinetic energy store is not changing. The 450 J is transferred to the thermal energy stores of the runners and the track. The track and sledge get slightly warmer.

Stored energy and energy transfers (P2.2n)

Two stores that change when forces do work are gravitational potential energy and kinetic energy. The specification lists their equations as ones to recall and apply:

• gravitational potential energy (J) = mass (kg) × gravitational field strength (N/kg) × height (m), so E = m × g × h
• kinetic energy (J) = ½ × mass (kg) × (speed (m/s))², so E = ½ × m × v²

In calculations, the question will give you g. This guide uses g = 10 N/kg. Using 9.8 N/kg is also common, and gives slightly smaller answers.

Worked example 5: lifting and stored energy

A builder lifts a 4.5 kg bucket straight up 3.2 m at a steady speed. Calculate the force needed, the work done and the gain in gravitational potential energy.

Force = weight = m × g = 4.5 × 10 = 45 N
W = F × s = 45 × 3.2 = 144 J
GPE gain = m × g × h = 4.5 × 10 × 3.2 = 144 J

The work done by the lifting force (144 J) equals the energy stored. The two methods give the same answer because lifting at steady speed uses a force equal to the weight, so F × s = m × g × h.

Worked example 6: kinetic energy and braking distance

A car of mass 1,200 kg travels at 15 m/s. Its brakes apply a constant resistive force of 6,000 N.

(a) Calculate the kinetic energy.

E = ½ × m × v²
E = 0.5 × 1200 × 15²
E = 0.5 × 1200 × 225
E = 135,000 J (135 kJ)

(b) The work done by the braking force must equal the energy removed from the kinetic energy store, so find the distance.

W = F × s, so s = W ÷ F
s = 135,000 ÷ 6,000
s = 22.5 m

The car stops in 22.5 m under braking. The energy ends up in thermal stores of the brakes, tyres and road.

Newton-metres and joules

One joule is the work done when a force of 1 newton moves an object 1 metre along the line of action of the force. So 1 N m = 1 J. If a question gives work in N m, you can write it in J with no change to the number. Take care with prefixes: 3.5 kJ = 3,500 J and 2 MJ = 2,000,000 J.

Power (P2.2o)

Power is the rate at which energy is transferred, which is the rate of doing work. A machine that transfers the same energy in less time has more power.

power (W) = work done (J) ÷ time (s)

In symbols, P = W ÷ t. One watt is one joule per second (1 W = 1 J/s). Also 1 kW = 1,000 W. Rearranged: W = P × t and t = W ÷ P.

Worked example 7: power from a climb

A student of mass 52 kg climbs a flight of stairs of vertical height 3.6 m in 6.5 s. Calculate the useful power she develops against gravity.

Work done = m × g × h = 52 × 10 × 3.6 = 1872 J
P = W ÷ t = 1872 ÷ 6.5
P = 288 W

Worked example 8: lifting machine

A crane lifts a 250 kg load through 12 m in 20 s.

W = m × g × h = 250 × 10 × 12 = 30,000 J
P = 30,000 ÷ 20 = 1,500 W = 1.5 kW

How long would the same crane take to do 24,000 J of work at this power?

t = W ÷ P = 24,000 ÷ 800 = 30 s   (for a motor rated at 800 W)

The 800 W figure is a different motor, given to practise rearranging for time.

Explaining power with examples

Two people may do the same work in different times. If two students each climb the same stairs, the one who gets to the top faster has the greater power, even though both do the same work. Power also depends on the work: a heavier person climbing in the same time has more power.

Power is not the same as energy. Energy is the amount transferred (J). Power is how fast it is transferred (W). A high-power device can still transfer a small amount of energy if it runs for a very short time.

Common errors

• Using the length of an object or the total path when the force is not along it. Use the distance moved along the line of action.
• Forgetting to convert units: kJ to J, km to m, minutes to seconds. Do this first.
• Giving the answer to a lifting question in N because you stopped at the weight. Check that you answered the quantity asked for.
• Saying “work is used up” or “energy is lost”. Say the energy is transferred to thermal stores of the surroundings.
• Writing W for both work and watts. Use the unit J for work and W for power, and write the equation in words as well as symbols.
• Mixing up power and energy in explanations. Power is a rate.

Next steps

Test yourself on this unit with the practice questions, then keep the revision notes open while you revise. For exam technique across the course, see the OCR GCSE Physics exam preparation page.

Official syllabus

OCR, GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0, July 2026, first assessment in 2018. Statements P2.2m, P2.2n and P2.2o.