OCR GCSE Physics A: Everyday speeds, reaction times and stopping distances (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 P8.1a to P8.1e in topic P8.1 Physics on the move: typical everyday speeds, estimating accelerations, converting units and computing rates, measuring human reaction times, and the factors that affect stopping distance. None of these five statements is shown in bold, so all of them can be assessed in both tiers.

The later P8.1 statements (P8.1f to P8.1i) are taught on other pages.

Related pages: OCR GCSE Physics hub, printable checklist, revision notes for this unit, practice questions for this unit and the wider OCR GCSE Physics forces guide. To find your gaps first, try the free 10-minute diagnostics.

What this unit covers

Statement	What you must be able to do
P8.1a	Recall typical speeds for wind and sound, and for walking, running, cycling and other transport
P8.1b	Estimate the magnitudes of everyday accelerations
P8.1c	Use ratios and proportional reasoning to convert units and compute rates, including non-SI to SI units
P8.1d	Explain methods of measuring human reaction times and recall typical results
P8.1e	Explain the factors that affect thinking distance, braking distance and overall stopping distance, and the safety implications

The specification suggests an investigation of reaction time using ruler drop experiments for P8.1d.

Typical everyday speeds (P8.1a)

The specification does not print a list of values, so the figures below are sensible approximate values to learn. In an answer, a value in the right region is what matters, not three significant figures.

Situation	Typical speed (m/s)	Roughly in other units
Walking	1.5	5 km/h
Running (jogging pace)	3	11 km/h
Cycling	6	20 km/h
Car in a town	13	30 mph
Car on a motorway	30	about 70 mph
Passenger aircraft cruising	250	900 km/h
Sound in air	340	1200 km/h
Light wind (breeze)	5	18 km/h
Storm-force wind	25 or more	90 km/h or more

Use these for sense-checking: a walking speed of 15 m/s means something has gone wrong.

Worked example 1: is the claim sensible?

A student says she walked 2.4 km to school in 8.0 minutes. Is that sensible?

1. Convert to SI units: 2.4 km = 2400 m and 8.0 min = 480 s.
2. Speed = distance ÷ time = 2400 ÷ 480 = 5.0 m/s.
3. Compare: a typical walking speed is about 1.5 m/s, so 5.0 m/s is more like a fast cyclist.
4. At 1.5 m/s the walk would take 2400 ÷ 1.5 = 1600 s, about 27 minutes.

Conclusion: the claim is not sensible for walking.

Estimating accelerations (P8.1b)

Acceleration is the change in velocity divided by the time taken:

a = Δv ÷ t, with Δv in m/s and t in s, gives a in m/s²

To estimate an everyday acceleration, pick a sensible change in speed and a sensible time from experience, then divide. Round heavily: the aim is the right order of magnitude.

Situation	Typical magnitude (m/s²)
Lift starting to move	about 1
Cyclist pulling away	about 1
Car pulling away normally	2 to 3
Sprinter at the start	3 to 5
Car braking hard in an emergency	6 to 8 (a deceleration)
Object in free fall near Earth	about 10

Worked example 2: estimate an acceleration

Estimate the acceleration of a car that goes from rest to 27 m/s in 9.0 s.

1. Change in velocity = 27 − 0 = 27 m/s.
2. Acceleration = 27 ÷ 9.0 = 3.0 m/s².
3. Compare with free fall (about 10 m/s²): this is roughly one third of that, which fits a quick but ordinary car.

A deceleration is a negative acceleration. Quote the magnitude when asked how large it is.

Ratios, proportion and unit conversion (P8.1c)

To convert units, multiply or divide by the right ratio. Always write the units at each step.

Conversion	Rule
mph to m/s	multiply by 1609 (metres in a mile), divide by 3600
m/s to km/h	× 3.6
km/h to m/s	÷ 3.6
minutes to seconds	× 60
hours to seconds	× 3600
cm to m	divide by 100
g to kg	divide by 1000

The 3.6 comes from 1000 m ÷ 3600 s. A speed in m/s is always the smaller number.

Worked example 3: km/h to m/s, then use proportion

A coach travels at a steady 108 km/h.

1. Speed in m/s = 108 ÷ 3.6 = 30 m/s.
2. Distance is directly proportional to time at constant speed. In 8.0 s the coach travels 30 × 8.0 = 240 m.

Worked example 4: mph to m/s

Convert 30 mph to m/s, given 1 mile = 1609 m.

1. Distance per hour = 30 × 1609 = 48,270 m.
2. Time = 3600 s.
3. Speed = 48,270 ÷ 3600 = 13.4 m/s, so 13 m/s to 2 significant figures.

Worked example 5: computing a rate

A runner’s pace is 5 minutes 30 seconds per kilometre. Find the speed in m/s.

1. Time for 1000 m = 5 × 60 + 30 = 330 s.
2. Speed = 1000 ÷ 330 = 3.0 m/s (2 s.f.).

A tap filling a 3.0 litre jug in 4.0 minutes delivers 3000 mL ÷ 240 s = 12.5 mL/s.

Measuring human reaction times (P8.1d)

Reaction time is the time between a stimulus (something seen, heard or felt) and the start of the response. It is a fraction of a second, so you need a method that avoids the delay in the person timing it.

Method 1: the ruler drop

1. A partner holds a 30 cm ruler vertically so that the zero mark is level with the top of your open thumb and finger. Your fingers are not touching the ruler.
2. Your partner drops the ruler without warning.
3. You catch it as quickly as you can.
4. Read the distance on the ruler at the top of your thumb.
5. Convert the distance to a time. The ruler is in free fall, so the longer it falls before you catch it, the longer your reaction time. A table or relationship is normally provided.

The table below is for a ruler in free fall from rest (g = 9.8 m/s²). It is shown as an example of the kind of conversion you may be given.

Distance dropped (cm)	Reaction time (s)
5	0.10
10	0.14
15	0.18
20	0.20
25	0.23
30	0.25

Method 2: computer-based tests

A computer shows a stimulus at a random moment and records the time until you click or press a key. The timing is electronic, which removes the human error in a stopwatch. With two people and a stopwatch, the timer’s own reaction time adds an error as large as the quantity being measured.

Improving reliability

• Repeat several times and take the arithmetic mean.
• Treat a result far from the others as an anomaly and consider repeating it.
• Drop from the same starting position each time, with the same hand.
• Change only one variable at a time, for example comparing with and without a distraction, and keep everything else the same.

Typical results

An alert person typically has a reaction time of about 0.2 s in a ruler drop. Longer values, up to around one second, occur when people are tired, distracted or facing a more complex decision, such as a driver.

Worked example 6: ruler drop results

Five catches give distances of 14.2, 17.0, 15.6, 13.8 and 16.4 cm.

1. Mean distance = (14.2 + 17.0 + 15.6 + 13.8 + 16.4) ÷ 5 = 77.0 ÷ 5 = 15.4 cm.
2. Using the table, 15 cm corresponds to about 0.18 s, so the reaction time is about 0.18 s.

Worked example 7: does a distraction matter?

A student takes four reaction times for each condition: with no distraction, 0.21, 0.23, 0.19 and 0.22 s; while reading a phone, 0.27, 0.30, 0.26 and 0.29 s.

1. Mean without distraction = 0.85 ÷ 4 = 0.2125 s.
2. Mean with distraction = 1.12 ÷ 4 = 0.28 s.
3. Difference = 0.28 − 0.2125 = 0.0675 s, about 0.07 s.
4. Percentage increase = 0.0675 ÷ 0.2125 × 100 = 32%.

Conclusion: the phone increased the mean reaction time, and the ranges do not overlap, so the evidence supports the idea. It is one person and four trials each, so a firm general claim would need more people and more repeats.

Stopping distance and safety (P8.1e)

Stopping distance = thinking distance + braking distance.

• Thinking distance: how far the vehicle travels while the driver is reacting, before any braking starts.
• Braking distance: how far the vehicle travels from the moment the brakes act until it is at rest.

During thinking distance the car moves at its original speed, so thinking distance = speed × reaction time. Doubling the speed doubles the thinking distance when the reaction time is unchanged.

Factor	Affects	Why
Speed	Both	More distance covered per second while reacting; more kinetic energy to remove while braking
Tiredness	Thinking	Slower reaction time
Alcohol or drugs	Thinking	Slower reaction time
Distraction, such as a phone	Thinking	Reaction time begins later or lasts longer
Wet, icy or leaf-covered road	Braking	Less friction between tyre and road, so a smaller maximum braking force
Worn tyres	Braking	Less grip, so a smaller maximum braking force
Worn or faulty brakes	Braking	Less braking force can be produced
Greater mass of vehicle or load	Braking	More kinetic energy at the same speed, so more work must be done to stop it with the same braking force

Alcohol, drugs, tiredness and distraction change reaction time, so they change thinking distance, not braking distance.

Why speed matters so much for braking

The brakes do work on the vehicle. Work done = braking force × braking distance, and this must equal the kinetic energy removed. Kinetic energy depends on the square of speed, so doubling the speed gives four times the kinetic energy. With the same braking force, the braking distance becomes four times as large.

Worked example 8: thinking distance

A car travels at 43.2 km/h. The driver’s reaction time is 0.85 s.

1. Speed = 43.2 ÷ 3.6 = 12 m/s.
2. Thinking distance = 12 × 0.85 = 10.2 m.
3. Alcohol lengthens the reaction time to 1.2 s: thinking distance = 12 × 1.2 = 14.4 m.
4. The increase is 4.2 m. The braking distance is unchanged, because the brakes and road have not changed.

Safety implications

• Speed limits are lower where pedestrians are likely, such as near schools, because the whole stopping distance is shorter at lower speeds.
• Drivers should leave a larger gap to the vehicle in front in rain or ice.
• Laws against drink-driving, drug-driving and using a phone at the wheel reduce reaction-time effects.
• Regular checks of brakes and tyres keep the braking force as large as the road allows.

Common errors

• Dividing km/h by 60 instead of 3.6.
• Putting minutes straight into a speed formula without converting to seconds.
• Saying that tiredness or alcohol increase braking distance.
• Using the ruler distance in cm as if it were a time.
• Taking one ruler drop as the result instead of a mean.
• Writing “braking distance doubles when speed doubles” instead of quadruples.

Next steps

Revise with the revision notes, then test yourself with the practice questions.

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.