Key Stage 4 science · Physics – Forces and motion

GCSE motion, acceleration and stopping distances: a guide for parents

This GCSE physics topic is about how and why things move. Students estimate everyday speeds, read and draw graphs of distance against time, link acceleration to the resultant force using Newton’s first law, tell weight apart from mass, and use all of it to understand thinking distance, braking distance and why speed matters so much on the road.

Motion, acceleration and road safety, explained

Speed is the distance travelled divided by the time taken. Students are expected to know typical values and to estimate sensibly: walking is about 1.5 metres per second, running about 3, cycling about 6, and sound travels through air at roughly 330 to 340 metres per second, which is why you see lightning before you hear thunder. On a distance-time graph, a flat line means stopped, a straight sloping line means constant speed, and a steeper line means faster. A curve means the speed is changing. The gradient of the line, the rise divided by the run, gives the speed.

Acceleration is the rate of change of velocity, in metres per second squared, found by dividing the change in velocity by the time taken. Students estimate everyday accelerations too: a car that goes from rest to 20 metres per second in 8 seconds has an average acceleration of 20 ÷ 8 = 2.5 m/s². Many courses also use velocity-time graphs, where the gradient gives the acceleration. Newton’s first law says that an object stays at rest, or keeps moving at a steady speed in a straight line, unless a resultant force acts on it. So a car at constant speed on a flat road has balanced forces, not zero forces: the engine’s driving force is matched by air resistance and friction. A resultant force causes acceleration, and the bigger the force or the smaller the mass, the bigger the acceleration. Newton’s second law puts this as force equals mass times acceleration.

Mass and weight are different things. Mass, in kilograms, is the amount of matter in an object and is the same everywhere. Weight is the force of gravity on that mass, measured in newtons, and is found by multiplying mass by the gravitational field strength. On Earth the field strength is about 9.8 N/kg, often rounded to 10, and on the Moon it is about 1.6 N/kg. An astronaut keeps the same mass on the Moon but weighs about a sixth as much.

On the roads, the total stopping distance is the thinking distance plus the braking distance. Thinking distance is how far the vehicle travels while the driver reacts, and is increased by tiredness, alcohol, drugs and distractions such as a phone. Braking distance is how far it travels once the brakes are applied, and is increased by high speed, a heavily loaded vehicle, worn brakes or tyres, and wet or icy roads. For the same braking force, braking distance depends on the speed squared, so doubling the speed makes it about four times longer. This is the physics behind speed limits and the advice to leave a larger gap in poor weather.

Words your child will hear

Speed: How far something travels each second, or each hour. Distance divided by time.
Acceleration: The rate at which velocity changes. Measured in metres per second squared (m/s²).
Resultant force: The overall force left when all the forces on an object are combined.
Weight: The force of gravity on an object, in newtons (N). Mass × gravitational field strength.
Gravitational field strength: The force on each kilogram of mass. About 9.8 N/kg on Earth.
Thinking distance: The distance travelled between a driver seeing a hazard and pressing the brake.
Braking distance: The distance travelled from when the brakes are applied until the vehicle stops.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about the speed of sound, and estimating speeds and accelerations in everyday contexts
Students should be taught about interpreting quantitatively graphs of distance, time, and speed
Students should be taught about acceleration caused by forces; Newton's First Law
Students should be taught about weight and gravitational field strength
Students should be taught about decelerations and braking distances involved on roads, and safety

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

A car of mass 1,000 kg travels at 20 m/s (about 45 mph). The driver’s reaction time is 0.7 s and, on a dry road, the car then decelerates at a steady 8 m/s² (a round figure for hard braking, chosen to keep the numbers simple). Find the stopping distance and the braking force, then find the new stopping distance if the speed is doubled to 40 m/s.

Thinking distance: the car keeps moving at constant speed while the driver reacts.
```
distance = speed × time
         = 20 × 0.7
         = 14 m
```
Time to stop once braking starts, using deceleration = change in speed ÷ time.
```
time = 20 ÷ 8
     = 2.5 s
```
Braking distance: the speed falls steadily from 20 to 0 m/s, so the average speed is 10 m/s. Many courses instead use the equation (final speed)² − (initial speed)² = 2 × acceleration × distance, which gives the same 25 m.
```
distance = 10 × 2.5
         = 25 m
```
Add them to get the stopping distance, then find the braking force from force = mass × acceleration.
```
14 + 25 = 39 m
F = 1,000 × 8 = 8,000 N
```
Now double the speed. Thinking distance is 40 × 0.7 = 28 m. Braking takes 40 ÷ 8 = 5 s at an average of 20 m/s, so the braking distance is 100 m.
```
28 + 100 = 128 m
```

At 20 m/s the stopping distance is 39 m (14 m thinking and 25 m braking), with a braking force of 8,000 N. At 40 m/s it is 128 m: the thinking distance doubles but the braking distance quadruples.

Common mistakes and how to help

Believing a constant force is needed to keep something moving at constant speed

In daily life, things do slow down when you stop pushing, because friction and air resistance act on them.

What helps: Think about a puck on ice or a spacecraft in deep space. With balanced forces an object keeps moving. A car’s engine force at constant speed is just balancing the resistive forces.

Using mass and weight as if they were the same

Shop scales give “weight” in kilograms, so the everyday word means mass.

What helps: Keep to the units. Kilograms are for mass and newtons for weight. A 50 kg student has weight 50 × 10 = 500 N on Earth but 50 × 1.6 = 80 N on the Moon, with the same mass.

Reading a graph as a picture of the journey

A hill-shaped distance-time graph looks like a hill, and students think the car went up and down.

What helps: Always read the axes first. On a distance-time graph, a flat line means stopped, not travelling level. A line that comes back down means the object is returning towards the start.

Assuming stopping distance is proportional to speed

Thinking distance is proportional to speed, so students expect the whole stopping distance to follow suit.

What helps: Calculate both parts for two speeds, as in the example. The braking distance quadruples when speed doubles, so small increases in speed make a big difference at higher speeds.

How to help at home

Measure your reaction time. One person holds a 30 cm ruler at the top, the other places a thumb and finger at the bottom without touching it. Drop the ruler without warning and note where it is caught. A catch at 20 cm is about 0.2 s. Ask what would make this longer on the road.
Estimate and time everyday speeds. Time your child walking, jogging or cycling over 20 metres of garden or park, well away from roads, and work out the speeds. Compare them with the typical values and ask which of them can be reliably estimated by eye.
Thunder counting. Next time there is a storm, watch from indoors and count the seconds between the flash and the bang. About 3 seconds means roughly 1 km. It makes the speed of sound feel real, and it is a good example of an estimate.
Draw a graph of a journey. Ask your child to sketch a distance-time graph of a trip to school or the shops, with stops and slow stretches, then read it back to you. Doing it the other way round, telling the story from a graph, is just as useful.
Talk about road safety. When you are a passenger in a car or bus, talk about why you leave a bigger gap in the rain and why speed limits are lower outside schools. Link it to thinking and braking distances, rather than only to rules.

Builds on

Key Stage 3: Speed and distance-time graphs
Key Stage 3: Forces, force diagrams and moments
Key Stage 3: Deformation, friction and Hooke’s Law
Key Stage 3: Pressure in fluids, floating and sinking
Key Stage 3: Balanced forces, equilibrium and motion

Everything in Key Stage 4 science

Updated 20 October 2026.