Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Explain the dangers caused by large decelerations and estimate the forces…
Full specification wording
Topic 2 – Motion and forces: 2.31 Explain the dangers caused by large decelerations and estimate the forces involved in typical situations on a public road
A large deceleration means velocity changes quickly, so the rate of change of momentum is large and the force on the vehicle and its occupants is large.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 30 of 32
Study notes
From the checked teaching notes for this specification point.
The idea
A large deceleration means velocity changes quickly, so the rate of change of momentum is large and the force on the vehicle and its occupants is large. Use F = mΔv/t: a 1200 kg car slowing from 20 m/s to rest in 0.5 s feels about 48 000 N, whereas the same stop over 5 s gives about 4800 N. Dangers include injury from sudden forces on the body, loss of control, skidding, and objects continuing forward. Seat belts and crumple zones lengthen the stopping time, reducing force. Estimate by choosing typical mass, speed and stopping time, then compute and state the answer with a sensible unit and size.
What earns the marks
- Uses the relationship between force, mass, change in velocity and stopping time to estimate a typical force.
- Explains that a shorter stopping time produces a larger decelerating force for the same change in velocity.
- Identifies a danger such as injury to occupants, loss of control or skidding caused by large deceleration.
- States that safety features lengthen the stopping time and so reduce the force on occupants.
Common mistakes and how to put them right
Confuses deceleration with a small force.
Put it right: A rapid stop involves a large force.
Omits the mass when estimating force.
Put it right: Include mass, change in velocity and time in the calculation.
Treats a longer stopping time as increasing the force.
Put it right: Recognise that a longer time reduces the force.
Exam tips
- Show the values you choose for mass, speed and stopping time before calculating.
- Give the estimated force a unit and check its size is realistic for a road vehicle.
- Link each safety feature to the increase in stopping time and the reduction in force.
Try the lesson
The start of the Topic 2 – Motion and forces: 2.31 Explain the dangers caused by large decelerations and estimate the forces involved in typical situations on a public road lesson, made from checked teaching notes for this specification point.
Have a go first
Which feature of a car is designed to reduce the force on passengers during a crash?
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