What you'll be able to do by the end
- Describe the energy stores and the ways energy is transferred
- Apply conservation of energy and calculate efficiency
- Calculate kinetic, gravitational potential and elastic potential energy
- Calculate power and work done
- Draw and interpret free body diagrams and calculate resultant forces
- Use the equations of motion
- Interpret distance–time and velocity–time graphs
- Apply Newton's three laws
- Calculate momentum and apply conservation of momentum
- Explain stopping distance and the factors affecting it
Most lost marks in this topic are unit errors, unconverted values and rearranged equations — not misunderstandings. Teach the method as hard as the content.
Part 1 — Working with equations
Before any content — this is the part that costs the marks.
The method that gets full marks
| Step | What to do |
|---|---|
| 1. Write the equation | Even if you rearrange it later. It's often worth a mark on its own |
| 2. Convert units | Everything into SI units before substituting |
| 3. Substitute | Numbers in, showing your working |
| 4. Calculate | |
| 5. Units | Every answer needs them |
If you get the arithmetic wrong but wrote the correct equation and substituted correctly, you usually get 2 out of 3.
If you write only an answer and it's wrong, you get 0.
Never do it in your head.
Units you must convert
| Given | Convert to | How |
|---|---|---|
| Grams | Kilograms | ÷ 1000 |
| Kilometres | Metres | × 1000 |
| Minutes | Seconds | × 60 |
| Hours | Seconds | × 3600 |
| km/h | m/s | ÷ 3.6 |
| Centimetres | Metres | ÷ 100 |
| Kilojoules | Joules | × 1000 |
Not converting grams to kilograms.
A 500 g mass is 0.5 kg. Using 500 makes every answer 1000 times too large.
Check every mass before you substitute. If your answer for the kinetic energy of a tennis ball comes out as 40,000 J, you've done it.
Part 2 — Energy
The energy stores
Energy is not created or destroyed — it is transferred between stores.
| Store | Example |
|---|---|
| Kinetic | Anything moving |
| Gravitational potential | Anything raised above the ground |
| Elastic potential | A stretched spring or elastic band |
| Thermal (internal) | A hot object |
| Chemical | Food, fuel, a battery |
| Nuclear | The nucleus of an atom |
| Magnetic | Two magnets held apart |
| Electrostatic | Two charges held apart |
Ways energy is transferred
| Pathway | Example |
|---|---|
| Mechanically | By a force doing work — pushing, lifting |
| Electrically | By a current |
| By heating | Conduction, convection |
| By radiation | Light, sound, infrared |
"Light energy" and "sound energy" are not stores. They are pathways — ways energy moves.
Modern specifications are strict about this. Write "energy is transferred by radiation" or "energy is transferred to the thermal store of the surroundings."
Conservation of energy
Energy cannot be created or destroyed. It can only be transferred, stored or dissipated.
Dissipated means spread out into the surroundings, usually to the thermal store — where it is no longer useful.
Efficiency
Worked example. A motor is supplied with 500 J and usefully transfers 350 J to the kinetic store.
(350 ÷ 500) × 100 = 70% efficient
The other 150 J is dissipated, usually as heat and sound.
No device is 100% efficient. Some energy is always dissipated.
Reducing unwanted transfers
| Method | Where |
|---|---|
| Lubrication | Reduces friction between moving parts |
| Insulation | Reduces energy transfer by conduction |
| Streamlining | Reduces air resistance |
| Cavity wall insulation | Reduces conduction and convection in buildings |
Part 3 — Energy calculations
Kinetic energy
½ × m × v² means square v first, then multiply.
½ × 1200 × 15 = 9000 ✗ ½ × 1200 × 225 = 135,000 ✓
And note: doubling the speed quadruples the kinetic energy. That's why speed matters so much in crashes.
Gravitational potential energy
Eₚ = 65 × 9.8 × 12 = 7644 J
Elastic potential energy
Eₑ = ½ × 40 × 0.15² = ½ × 40 × 0.0225 = 0.45 J
Work done
One joule is the work done when a force of one newton moves an object one metre.
W = 250 × 8 = 2000 J
Power
Power is the rate of energy transfer. One watt is one joule per second.
P = 6000 ÷ 20 = 300 W
Two people carry the same load up the same stairs. They do the same work and transfer the same energy.
The one who does it faster has more power.
Power is not "how much" — it's "how fast."
Part 4 — Forces
Scalars and vectors
| Has | Examples | |
|---|---|---|
| Scalar | Magnitude only | Distance, speed, mass, energy, time |
| Vector | Magnitude and direction | Displacement, velocity, force, acceleration, momentum |
Distance is scalar. Displacement is vector.
Speed is scalar. Velocity is vector.
Mass is scalar. Weight is vector — it's a force.
A runner completing a 400 m lap has travelled a distance of 400 m and a displacement of zero.
Contact and non-contact forces
| Contact | Non-contact |
|---|---|
| Friction | Gravitational |
| Air resistance | Electrostatic |
| Tension | Magnetic |
| Normal contact force |
Weight
Mass is the amount of matter — kilograms, same everywhere.
Weight is the force of gravity on that mass — newtons, changes with location.
Resultant force
The single force that has the same effect as all the forces acting.
Resultant = 4000 − 1500 = 2500 N forwards
If the resultant force is zero, the object is in equilibrium — either stationary or moving at constant velocity.
Part 5 — Newton's laws
First law
An object will remain at rest, or moving at constant velocity, unless acted on by a resultant force.
- Stationary object, zero resultant force → stays still
- Moving object, zero resultant force → continues at the same speed in the same direction
Because the driving force is balancing friction and air resistance, not accelerating the car.
Zero resultant force does not mean no forces. It means they cancel.
Second law
a = F ÷ m = 900 ÷ 600 = 1.5 m/s²
Inertial mass is a measure of how difficult it is to change an object's velocity — the ratio of force to acceleration.
Third law
Whenever two objects interact, the forces they exert on each other are equal and opposite.
The two forces in a Newton's third law pair act on different objects.
You push the ground, the ground pushes you. Only one of those acts on you, so you move.
Forces only cancel when they act on the same object.
Part 6 — Motion
Speed, velocity and acceleration
a = (24 − 8) ÷ 4 = 16 ÷ 4 = 4 m/s²
The equation without time
v² − 0² = 2 × 3 × 50 = 300 v = √300 = 17.3 m/s
Typical values worth knowing
| Speed | |
|---|---|
| Walking | ~1.5 m/s |
| Running | ~3 m/s |
| Cycling | ~6 m/s |
| Car in a town | ~13 m/s (30 mph) |
| Sound in air | ~330 m/s |
Distance–time graphs
| Feature | Meaning |
|---|---|
| Gradient | Speed |
| Horizontal line | Stationary |
| Straight sloped line | Constant speed |
| Curved line getting steeper | Accelerating |
| Curved line getting shallower | Decelerating |
To find speed at a point on a curve: draw a tangent and find its gradient.
Velocity–time graphs
| Feature | Meaning |
|---|---|
| Gradient | Acceleration |
| Horizontal line | Constant velocity |
| Straight sloped line | Constant acceleration |
| Line sloping down | Deceleration |
| Area under the graph | Distance travelled |
On a distance–time graph, the gradient is speed.
On a velocity–time graph, the gradient is acceleration and the area is distance.
Students routinely find the area under a distance–time graph, which means nothing at all. Check which graph you're looking at first.
Terminal velocity
A falling object:
- Initially, weight is greater than air resistance → resultant force downwards → it accelerates
- As speed increases, air resistance increases
- Eventually air resistance equals weight → resultant force is zero
- The object falls at constant terminal velocity
Opening a parachute increases air resistance sharply, so the resultant force is now upwards, the object decelerates, air resistance falls again, and a new lower terminal velocity is reached.
Part 7 — Stopping distance
| Distance | What it is | Affected by |
|---|---|---|
| Thinking | Distance travelled during reaction time | Tiredness · alcohol · drugs · distractions · speed |
| Braking | Distance travelled while braking | Speed · road conditions (wet, icy) · tyre condition · brake condition · vehicle mass |
Speed affects thinking distance proportionally — twice the speed, twice as far during your reaction time.
Speed affects braking distance as a square — because kinetic energy depends on v². Twice the speed means four times the braking distance.
This is why speed limits matter more than people think, and it's a favourite exam question.
Braking and energy
When brakes are applied, work is done by friction between the brakes and the wheels. The kinetic store of the vehicle is transferred to the thermal store of the brakes, which get hot.
Greater speed requires greater braking force for the same braking distance — which risks brakes overheating and loss of control.
Part 8 — Momentum
Momentum is a vector — direction matters.
Conservation of momentum
In a closed system, the total momentum before an event equals the total momentum after.
Force and momentum
Increasing the time over which momentum changes reduces the force.
| Feature | How it works |
|---|---|
| Air bag | Increases the time for the body to stop |
| Crumple zone | Deforms, extending the collision time |
| Seat belt | Stretches slightly, increasing stopping time |
| Crash mat | Compresses, extending the time |
| Bending your knees when landing | Increases the time to stop |
Same change in momentum. Longer time. Smaller force. That's the whole principle, and it answers every safety-feature question on the paper.
Exam-style questions
Answer all questions. Marks are shown in brackets. Total: 76 marks.
Question 1
A lamp is supplied with 60 J of energy each second and usefully transfers 9 J as light.
(Total 9 marks)
Question 2
(Total 8 marks)
Question 3
A crane lifts a 250 kg load through 15 m in 30 seconds. Take g = 9.8 N/kg.
(Total 7 marks)
Question 4
A car has a driving force of 3200 N and experiences resistive forces of 3200 N.
(Total 8 marks)
Question 5
(Total 9 marks)
Question 6
A velocity–time graph shows a vehicle accelerating uniformly from 0 to 20 m/s in 8 seconds, then travelling at a constant 20 m/s for 12 seconds.
(Total 7 marks)
Question 7
(Total 7 marks)
Question 8
(Total 12 marks)
Question 9
(Total 9 marks)
TOTAL FOR PAPER: 76 MARKS
Mark scheme
Question 1
(a) 3 — any three: kinetic · gravitational potential · elastic potential · thermal · chemical · nuclear · magnetic · electrostatic.
| (b)(i) 2. (9 ÷ 60) × 100 | 1 |
| = 15% | 1 |
| (b)(ii) 2. 51 J is dissipated | 1 |
| to the thermal store of the surroundings | 1 |
| (c) 2. Some energy is always dissipated to the surroundings | 1 |
| usually to thermal stores through friction or heating, and is no longer useful | 1 |
Question 2
| (a) 3. Eₖ = ½ × m × v² | 1 |
| = ½ × 1400 × 20² = ½ × 1400 × 400 | 1 |
| = 280,000 J (280 kJ) | 1 |
| (b) 2. It is four times greater | 1 |
| because kinetic energy is proportional to velocity squared, and doubling v quadruples v² | 1 |
| (c) 3. 600 g = 0.6 kg | 1 |
| Eₚ = 0.6 × 9.8 × 2.5 | 1 |
| = 14.7 J | 1 |
Failing to convert grams gives 14,700 J. Award 1 for method only.
Question 3
| (a)(i) 3. Force needed = weight = 250 × 9.8 = 2450 N | 1 |
| W = F × d = 2450 × 15 | 1 |
| = 36,750 J | 1 |
| (a)(ii) 2. P = 36750 ÷ 30 | 1 |
| = 1225 W | 1 |
| (b) 2. Energy is the total amount transferred, measured in joules | 1 |
| Power is the rate at which energy is transferred, measured in watts | 1 |
Question 4
| (a) 3. A scalar has magnitude only | 1 |
| a vector has magnitude and direction | 1 |
| any correct examples, e.g. speed is scalar and velocity is vector | 1 |
(b)(i) 1 — 0 N.
| (b)(ii) 2. The car continues at constant velocity | 1 |
| because with zero resultant force there is no acceleration | 1 |
| (c) 2. An object remains at rest or moving at constant velocity | 1 |
| unless acted on by a resultant force | 1 |
Question 5
| (a) 3. a = F ÷ m | 1 |
| = 1500 ÷ 1000 | 1 |
| = 1.5 m/s² | 1 |
| (b) 3. a = (v − u) ÷ t | 1 |
| = (10 − 4) ÷ 3 | 1 |
| = 2 m/s² | 1 |
| (c) 3. v² − u² = 2as | 1 |
| v² = 2 × 2.5 × 80 = 400 | 1 |
| v = 20 m/s | 1 |
Question 6
| (a) 2. a = 20 ÷ 8 | 1 |
| = 2.5 m/s² | 1 |
| (b) 4. Distance = area under the graph | 1 |
| Triangle: ½ × 8 × 20 = 80 m | 1 |
| Rectangle: 12 × 20 = 240 m | 1 |
| Total = 320 m | 1 |
(c) 1 — Speed.
Question 7
| (a) 4. Initially weight is greater than air resistance, so there is a resultant downward force and the skydiver accelerates | 1 |
| As speed increases, air resistance increases | 1 |
| The resultant force decreases, so acceleration decreases | 1 |
| When air resistance equals weight, the resultant force is zero and the skydiver falls at constant terminal velocity | 1 |
| (b) 3. Air resistance increases sharply | 1 |
| Air resistance is now greater than weight, so there is a resultant upward force and the skydiver decelerates | 1 |
| As speed falls, air resistance falls until it again equals weight, giving a new lower terminal velocity | 1 |
Question 8
(a) 1 — thinking distance + braking distance.
(b) 4 — thinking (any two): tiredness · alcohol · drugs · distraction · speed. Braking (any two): speed · wet or icy road · worn tyres · worn brakes · greater mass.
| (c) 4. Thinking distance is proportional to speed, so it doubles | 1 |
| Braking distance depends on kinetic energy | 1 |
| Kinetic energy is proportional to v², so doubling the speed quadruples it | 1 |
| Braking distance therefore quadruples, so the total more than doubles | 1 |
| (d) 3. Work is done by friction between the brakes and the wheels | 1 |
| Energy is transferred from the kinetic store of the vehicle | 1 |
| To the thermal store of the brakes, which increase in temperature | 1 |
Question 9
| (a) 2. p = m × v = 1200 × 15 | 1 |
| = 18,000 kg m/s | 1 |
| (b) 4. Momentum before = (3 × 4) + (1 × 0) = 12 kg m/s | 1 |
| Momentum is conserved, so momentum after = 12 kg m/s | 1 |
| Combined mass = 4 kg, so 4v = 12 | 1 |
| v = 3 m/s | 1 |
| (c) 3. The air bag increases the time taken for the passenger's momentum to change to zero | 1 |
| Force = change in momentum ÷ time | 1 |
| A longer time means a smaller force acting on the passenger, so less injury | 1 |
Common mistakes in this topic
Not converting grams to kilograms. Every answer comes out 1000 times too big. Check every mass.
Not squaring v in kinetic energy. ½mv² means square the velocity first.
Finding the area under a distance–time graph. It means nothing. Area is only meaningful under a velocity–time graph.
Confusing mass and weight. Kilograms and newtons.
Saying "there are no forces" when the resultant is zero. The forces are balanced, not absent.
Saying Newton's third law pairs cancel. They act on different objects.
"Light energy" and "sound energy." These are pathways, not stores.
Omitting units. A number without units is usually not a complete answer.
Teaching notes
Teach the equation method before any content. Write it, convert, substitute, calculate, units. Drilled properly it earns partial credit on every calculation for the rest of the course, and it turns a wrong answer worth 0 into a wrong answer worth 2.
The grams-to-kilograms conversion deserves a starter every lesson for a fortnight. It is the single biggest source of lost marks in GCSE Physics and it is entirely preventable.
v² is the idea that explains stopping distances. Doubling speed quadruples kinetic energy and quadruples braking distance. Students who understand this can answer road-safety questions from first principles instead of memorising a table.
The two graph types must be distinguished explicitly and repeatedly. Gradient of distance–time is speed. Gradient of velocity–time is acceleration, and the area is distance. Ask "which graph am I looking at?" before every question.
Terminal velocity is best taught as a sequence, not a definition. Weight greater → accelerates → air resistance increases → forces balance → constant velocity. Four steps, four marks, and it works for the parachute question too.
All the safety-feature questions have one answer. Increase the time, reduce the force. Air bags, crumple zones, seat belts, crash mats, bending your knees. Teach the principle once and students can answer any version of it.
Written for GCSE Physics, all major UK boards. Check your specification — momentum and the v² − u² = 2as equation are Higher Tier on most boards. Reviewed 2026.