AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
(Physics only) Students should be able to estimate how the distance for a…
Full specification wording
Stopping distance: (Physics only) Students should be able to estimate how the distance for a vehicle to make an emergency stop varies over a range of speeds typical for that vehicle.
An emergency stop can be estimated rather than measured.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 3 of 4
Study notes
From the checked teaching notes for this specification point.
The idea
An emergency stop can be estimated rather than measured. Take a reaction time of about 0.5 s to 0.7 s and work out thinking distance = speed × reaction time. Then scale the braking distance with the square of the speed from a known value: if a car needs 6 m to brake from 10 m/s, it needs about 24 m from 20 m/s and about 54 m from 30 m/s. Adding the two gives roughly 12 m at 10 m/s, 36 m at 20 m/s and 72 m at 30 m/s, so tripling the speed makes the stop about six times longer. Quote estimates to one or two significant figures in metres, and state the reaction time you assumed. These figures assume the hardest braking a dry road allows; on a wet or icy surface the friction is smaller, so every braking estimate must be increased.
What earns the marks
- A stated reaction time in the typical range 0.2 s to 0.9 s.
- Calculating the thinking distance as speed × reaction time.
- Scaling the braking distance with the square of the speed.
- Adding the two parts to give an estimated stopping distance in metres.
- The braking distance scaled correctly, such as about 24 m at 20 m/s scaled from 6 m at 10 m/s.
Common mistakes and how to put them right
Scaling the braking distance in direct proportion to speed instead of to its square.
Using a speed in mph inside a calculation whose answer is wanted in metres.
Assuming a reaction time of several seconds, well outside the typical range.
Giving only the braking part and calling it the emergency stopping distance.
Exam tips
- Write down every assumed value with its unit before you calculate; estimates are marked on the method.
- Use 30 mph ≈ 13 m/s and 70 mph ≈ 31 m/s to move between road speeds and physics units.
- Sanity check the answer against the typical value that a car takes about 36 m to stop from 20 m/s.
Try the lesson
The start of the Stopping distance: (Physics only) Students should be able to estimate how the distance for a vehicle to make an emergency stop varies over a range of speeds typical for that vehicle. lesson, made from checked teaching notes for this specification point.
Have a go first
A car travelling at 10 m/s has a thinking distance of 6 m and a braking distance of 8 m. Estimate its total stopping distance if it travels at 20 m/s under the same conditions.
Listen
An emergency stop can be estimated rather than measured. Take a reaction time of about 0.5 s to 0.7 s and work out thinking distance = speed × reaction time. Then scale the braking distance with the square of the speed from a known value: if a car needs 6 m to brake from 10 m/s, it needs about 24 m from 20 m/s and about 54 m from 30 m/s. Adding the two gives roughly 12 m at 10 m/s, 36 m at 20 m/s and 72 m at 30 m/s, so tripling the speed makes the stop about six times longer. Quote estimates to one or two significant figures in metres, and state the reaction time you assumed. These figures assume the hardest braking a dry road allows; on a wet or icy surface the friction is smaller, so every braking estimate must be increased. A strong answer earns marks for these points. a stated reaction time in the typical range 0.2 s to 0.9 s. calculating the thinking distance as speed × reaction time. scaling the braking distance with the square of the speed. adding the two parts to give an estimated stopping distance in metres. Exam tip: Write down every assumed value with its unit before you calculate; estimates are marked on the method.
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