AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
(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.
The method uses F = m × a, where a is the deceleration.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 5 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The method uses F = m × a, where a is the deceleration. The estimate should be given to a reasonable number of significant figures and the direction of the force should be opposite to the motion. This skill is assessed by checking that the method is physically correct and the numerical answer is plausible for a public-road situation.
What earns the marks
- Select realistic values for the mass of the vehicle, its initial speed and the time taken to stop, for example a car of mass 1200 kg stopping from 13 m/s in 2.0 s.
- Calculate the deceleration using a = Δv ÷ t, ensuring the change in velocity is in m/s and the time is in s.
- Substitute the mass and deceleration into F = m × a to estimate the resultant force, and state the unit as N.
- Comment on whether the estimated force is reasonable for the situation, and note that the force acts opposite to the direction of motion.
Common mistakes and how to put them right
Using the speed in km/h directly in the calculation.
Put it right by converting to m/s before finding the deceleration.
Forgetting to divide by the stopping time and using the change in velocity as the deceleration.
Put it right by applying a = Δv ÷ t.
Giving an answer with unrealistic precision, such as many decimal places.
Put it right by rounding to a sensible number of significant figures for an estimate.
Exam tips
- Write down the values you choose for mass, speed and time before calculating, so the examiner can follow your estimate.
- Show the deceleration calculation separately from the force calculation to make the method clear.
- Check that your final force is in newtons and is plausible; a force of a few thousand newtons is typical for a car braking hard.
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
Which of the following is the most reasonable estimate for the average braking force acting on a typical family car during an emergency stop on a public road?
Listen
The method uses F = m × a, where a is the deceleration. The estimate should be given to a reasonable number of significant figures and the direction of the force should be opposite to the motion. This skill is assessed by checking that the method is physically correct and the numerical answer is plausible for a public-road situation. A strong answer earns marks for these points. Select realistic values for the mass of the vehicle, its initial speed and the time taken to stop, for example a car of mass 1200 kg stopping from 13 m/s in 2.0 s. Calculate the deceleration using a = Δv ÷ t, ensuring the change in velocity is in m/s and the time is in s. Substitute the mass and deceleration into F = m × a to estimate the resultant force, and state the unit as N. Comment on whether the estimated force is reasonable for the situation, and note that the force acts opposite to the direction of motion. A common slip: Using the speed in km/h directly in the calculation. Instead, converting to m/s before finding the deceleration. Exam tip: Write down the values you choose for mass, speed and time before calculating, so the examiner can follow your estimate.
See it
Put it in order
Put these steps in the order they happen.
- 1. Calculate deceleration using a = Δv ÷ t
- 2. Convert speed to m/s if needed
- 3. Choose realistic values: mass, initial speed, stopping time
- 4. Round to sensible significant figures and check plausibility
- 5. State force in N, acting opposite to motion
- 6. Substitute mass and deceleration into F = m × a
Key words
Say what each word means before you turn the card.
Estimating physical quantities
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Force, mass and acceleration
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Deceleration of vehicles
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Significant figures and units
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See every definition
- Estimating physical quantities
- Choosing sensible values for mass, initial speed and stopping time, then giving the answer to a reasonable number of significant figures.
- Force, mass and acceleration
- The resultant force is estimated by substituting the mass and the deceleration into F = m × a, with the unit stated as N.
- Deceleration of vehicles
- The change in velocity divided by the stopping time, calculated using a = Δv ÷ t with the change in velocity in m/s and the time in s.
- Significant figures and units
- An estimate should be rounded to a sensible number of significant figures, with units converted if needed and the force given in newtons.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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