OCR GCSE Physics A: Everyday speeds, reaction times and stopping distances (J249) – Revision Notes

These notes are for final revision of statements P8.1a to P8.1e of the OCR GCSE (9-1) Gateway Science Physics A specification (J249), Version 5.0, July 2026, first assessment in 2018. None of the five statements is Higher Tier only, so all of it is for both tiers. For full explanations and worked examples, start with the study guide.

Links: OCR GCSE Physics hub, printable checklist, practice questions, free 10-minute diagnostics.

Typical speeds (P8.1a)

Learn these as approximate values. A value in the right region is what counts.

Situation	Speed (m/s)
Walking	1.5
Running	3
Cycling	6
Car in town (30 mph)	13
Car on motorway	30
Aircraft cruising	250
Sound in air	340
Breeze	5
Storm-force wind	25 or more

Sense-check: walking is about 5 km/h, town traffic is a few tens of km/h, and sound in air is faster than any road vehicle. A wind speed in m/s multiplied by 3.6 gives km/h.

Estimating accelerations (P8.1b)

Equation	Meaning
a = Δv ÷ t	a in m/s², Δv in m/s, t in s

Method in steps:

1. Choose a sensible start speed and end speed.
2. Choose a sensible time.
3. Divide the change in speed by the time.
4. Round to one significant figure and compare with a known value.

Situation	Magnitude (m/s²)
Lift, cyclist pulling away	about 1
Car pulling away	2 to 3
Sprinter at start	3 to 5
Emergency braking, car	6 to 8
Free fall	about 10

Worked reminder: a train goes from rest to 20 m/s in 40 s. a = 20 ÷ 40 = 0.5 m/s².

Unit conversion, ratios and rates (P8.1c)

Convert	Rule
mph to m/s	× 1609, then ÷ 3600
m/s to km/h	× 3.6
km/h to m/s	÷ 3.6
min to s	× 60
h to s	× 3600
km to m	× 1000
cm to m	÷ 100
g to kg	÷ 1000

Method in steps:

1. Write the quantity with its unit.
2. Multiply or divide by the ratio, keeping units visible.
3. Check the size: does the answer sit in a sensible range for the situation?
4. Round sensibly to 2 or 3 significant figures.

Proportion: at constant speed, distance is directly proportional to time. Twice the time gives twice the distance. A rate is a quantity per unit time, such as metres per second or litres per minute.

Worked reminder: a wind of 25 m/s is 25 × 3.6 = 90 km/h. A cyclist rides 18 km in 45 minutes. Distance = 18,000 m, time = 2700 s, speed = 18,000 ÷ 2700 = 6.7 m/s (2 s.f.).

Measuring reaction time (P8.1d)

Reaction time: the delay between a stimulus and the start of the response.

Method	How it works	Main strengths and weaknesses
Ruler drop	Ruler dropped without warning; distance caught converts to time	Cheap and quick; limited by how precisely the ruler is read, and the dropper may give clues
Computer test	Computer records time between stimulus and click	Electronic timing; no timer error; needs equipment
Two people, stopwatch	One person starts and stops the clock	Poor: the timer’s own reaction time is as large as the value being measured

Ruler-drop steps:

1. Hold the ruler with its zero level with the top of the thumb, with no contact.
2. Drop without warning.
3. Read the mark at the top of the thumb.
4. Convert the distance to a time using the table or relationship provided.
5. Repeat and take the mean.

Reliability checklist: repeat; calculate the arithmetic mean; check for anomalies; same start position; same hand; change only one variable.

Typical results: about 0.2 s for an alert person on a ruler drop; longer, up to around one second, when tired, distracted or facing a complex decision.

Worked reminder: distances 12, 15, 14 and 15 cm give a mean of 56 ÷ 4 = 14 cm, which is a reaction time of roughly 0.17 s from a free-fall table (14 cm falls in about 0.17 s).

Stopping distance and safety (P8.1e)

Stopping distance = thinking distance + braking distance.

Thinking distance = speed × reaction time.

Factor	Thinking distance	Braking distance
Higher speed	increases	increases (much more)
Tiredness	increases	no effect
Alcohol or drugs	increases	no effect
Distraction	increases	no effect
Wet or icy road	no effect	increases
Worn tyres	no effect	increases
Worn brakes	no effect	increases
Greater mass	no effect	increases

Why braking distance rises with speed: kinetic energy depends on speed squared. With the same braking force, work done = force × distance, so double the speed gives four times the braking distance.

Why the road surface matters: less friction between the tyre and the road means a smaller maximum braking force, so the vehicle takes more distance to lose the same kinetic energy.

Safety implications to link to each factor:

• Lower speed limits where pedestrians are present.
• Bigger gaps to the vehicle ahead in wet or icy conditions.
• Rules against drink-driving, drug-driving and phone use.
• Checks of tyres and brakes.

Worked reminder for thinking distance: a driver at 90 km/h has a reaction time of 0.60 s. Speed = 90 ÷ 3.6 = 25 m/s. Thinking distance = 25 × 0.60 = 15 m. If the driver is distracted and the reaction time rises to 0.90 s, the thinking distance becomes 22.5 m, which is 7.5 m further.

Method for an “explain” question on stopping distance:

1. Say which distance is affected, thinking or braking.
2. Name the cause: reaction time for thinking distance; braking force or kinetic energy for braking distance.
3. Link the cause to the factor given.
4. State the consequence for stopping distance and safety.

Must-know distinctions

• Thinking distance depends on the driver; braking distance depends on the vehicle and the road.
• Speed affects both distances, but not in the same way: thinking distance doubles, braking distance quadruples.
• Time to convert: seconds in SI calculations, not minutes or hours.
• Mean, not a single value, for reaction time.

Quick self-test

1. State a typical cycling speed in m/s.
2. Convert 144 km/h to m/s.
3. Convert 25 m/s to km/h.
4. Convert 20 mph to m/s, given 1 mile = 1609 m. Give 2 significant figures.
5. A scooter goes from rest to 15 m/s in 6.0 s. Calculate its acceleration.
6. Four reaction times are 0.24, 0.20, 0.28 and 0.24 s. Calculate the mean.
7. A car travels at 20 m/s and the driver’s reaction time is 0.50 s. Calculate the thinking distance.
8. A jogger runs 3.0 km in 15 minutes. Calculate the speed in m/s, to 2 significant figures.
9. State two factors that increase thinking distance but not braking distance.
10. A car’s speed doubles. State what happens to its braking distance for the same braking force, and why.
11. Give one reason why a computer reaction test is better than two people using a stopwatch.

Answers

1. About 6 m/s (accept values from about 4 to 8 m/s).
2. 144 ÷ 3.6 = 40 m/s.
3. 25 × 3.6 = 90 km/h.
4. 20 × 1609 ÷ 3600 = 8.9 m/s.
5. 15 ÷ 6.0 = 2.5 m/s².
6. 0.96 ÷ 4 = 0.24 s.
7. 20 × 0.50 = 10 m.
8. 3000 m ÷ 900 s = 3.3 m/s.
9. Any two from tiredness, alcohol, drugs, distraction.
10. It becomes four times as large, because kinetic energy depends on speed squared, and the same force has to do four times the work.
11. The timing is done electronically, so there is no extra human reaction time in the measurement.

Where marks are usually lost

• Dividing km/h by 60 or 10 instead of 3.6.
• Leaving minutes or hours in a speed calculation.
• Placing alcohol, tiredness or distraction under braking distance.
• Placing wet roads, worn tyres or worn brakes under thinking distance.
• Saying braking distance “doubles” when speed doubles.
• Giving one ruler drop as the reaction time, with no mean.
• Writing “faster reaction” when the stem asks about the effect of tiredness, which makes the reaction slower.
• Estimating an acceleration with an unreasonable time, such as 0 to 27 m/s in 1 s for a car.
• Not linking a factor to its effect: a wet road lowers friction and so the braking force, and a heavier vehicle has more kinetic energy to remove.
• Forgetting the unit, or giving m when the question asks for a time in s.

Official syllabus

OCR, GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0, July 2026, first assessment in 2018. Statements P8.1a to P8.1e.