AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
(HT only) estimate the forces involved in the deceleration of road…
Full specification wording
Factors affecting braking distance 2: (HT only) estimate the forces involved in the deceleration of road vehicles in typical situations on a public road.
Higher-tier students estimate the braking force on a road vehicle by combining Newton's second law with a typical deceleration.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 5 of 5
Study notes
From the checked teaching notes for this specification point.
The idea
Higher-tier students estimate the braking force on a road vehicle by combining Newton's second law with a typical deceleration. For a car of mass 1200 kg slowing from 20 m/s to rest in 40 m, first find deceleration using v² = u² + 2as: 0 = 20² + 2a × 40, so a = −5 m/s². The magnitude is 5 m/s². Then F = ma = 1200 × 5 = 6000 N. The negative sign shows the force opposes motion. Typical public-road decelerations range from about 4 m/s² for gentle braking to roughly 8 m/s² for emergency braking on dry tarmac, so estimated forces are of the order of thousands of newtons. The estimate depends on assumed mass, speed and braking distance, so state those assumptions clearly.
What earns the marks
- Selects a realistic mass for the vehicle, for example 1000 kg to 1500 kg for a family car, and states it as an assumption.
- Uses v² = u² + 2as, or an equivalent method, to obtain the deceleration from a chosen initial speed and braking distance.
- Applies F = ma with the deceleration magnitude to calculate the braking force in newtons.
- Recognises that the force acts opposite to the direction of motion and may be quoted as negative.
- Justifies the estimate by comparing the assumed deceleration with typical road values, roughly 4 m/s² to 8 m/s².
- Shows that a larger speed or shorter braking distance requires a larger force, since F is proportional to a.
Common mistakes and how to put them right
Using the speed instead of the deceleration in F = ma.
Put it right: Find a from the motion equation first, then multiply by mass.
Forgetting to square the initial speed in v² = u² + 2as.
Put it right: Write 20² = 400 before rearranging.
Using stopping distance instead of braking distance to find deceleration.
Put it right: Use only the braking distance.
Exam tips
- Write down the assumed mass, initial speed and braking distance before calculating so the examiner can follow your estimate.
- Keep the negative sign or state the direction of the force, because deceleration is a vector quantity.
- Check the size of your answer: a few thousand newtons is sensible for a car, while a few newtons or millions of newtons suggests an error.
Try the lesson
The start of the Factors affecting braking distance 2: (HT only) estimate the forces involved in the deceleration of road vehicles in typical situations on a public road. lesson, made from checked teaching notes for this specification point.
Have a go first
Without being given numerical data, estimate the braking force on a typical small car during a normal, controlled stop on a public road. Which value is the most reasonable?
Listen
Higher-tier students estimate the braking force on a road vehicle by combining Newton's second law with a typical deceleration. For a car of mass 1200 kg slowing from 20 m/s to rest in 40 m, first find deceleration using v² = u² + 2as: 0 = 20² + 2a × 40, so a = −5 m/s². The magnitude is 5 m/s². Then F = ma = 1200 × 5 = 6000 N. The negative sign shows the force opposes motion. Typical public-road decelerations range from about 4 m/s² for gentle braking to roughly 8 m/s² for emergency braking on dry tarmac, so estimated forces are of the order of thousands of newtons. The estimate depends on assumed mass, speed and braking distance, so state those assumptions clearly. A strong answer earns marks for these points. Selects a realistic mass for the vehicle, for example 1000 kg to 1500 kg for a family car, and states it as an assumption. Uses v² = u² + 2as, or an equivalent method, to obtain the deceleration from a chosen initial speed and braking distance. Applies F = ma with the deceleration magnitude to calculate the braking force in newtons. Recognises that the force acts opposite to the direction of motion and may be quoted as negative. A common slip: Using the speed instead of the deceleration in F = ma: the error is substituting 20 m/s directly. Instead, find a from the motion equation first, then multiply by mass. Exam tip: Write down the assumed mass, initial speed and braking distance before calculating so the examiner can follow your estimate.
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