What affects a car's stopping distance, and how do safety features reduce injury?
Stopping distance as thinking distance plus braking distance, the factors that affect each, the energy and force involved in braking, and how safety features reduce the force on occupants by increasing the collision time.
A focused answer to OCR Gateway GCSE Physics A topic P8 on vehicle safety and stopping distances, covering stopping distance as thinking plus braking distance, the factors affecting each, the energy and force in braking, and how safety features reduce the force on occupants by increasing the collision time.
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What this topic is asking
OCR wants you to define stopping distance as thinking distance plus braking distance, explain the factors that affect each, link braking to energy and force, and explain how safety features reduce injury by increasing the collision time. This is part of topic P8.1 (Physics on the move) of the OCR Gateway Physics A (J249) specification.
Stopping distance
This relationship is given on the data sheet. Both parts increase with speed, so the total stopping distance grows quickly as a car goes faster.
Factors affecting thinking distance
Factors affecting braking distance
Energy and force in braking
When a car brakes, the work done by the braking force transfers the car's kinetic energy to thermal energy in the brakes (which get hot). Because the kinetic energy is and depends on the speed squared, doubling the speed gives four times the kinetic energy and so roughly four times the braking distance for the same braking force. A larger braking force can stop a car in a shorter distance, but a very large force can cause the car to skid or the occupants to be thrown forward dangerously.
Safety features
Try this
Q1. State two factors that increase a driver's thinking distance. [2 marks]
- Cue. Higher speed and a longer reaction time (for example from tiredness, alcohol, drugs or distraction).
Q2. State the energy transfer that happens in the brakes when a car slows down. [1 mark]
- Cue. The car's kinetic energy is transferred to thermal energy in the brakes (which get hot).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksDefine stopping distance, and explain two factors that increase the thinking distance and two that increase the braking distance of a car.Show worked answer →
A P8 question worth four marks. The stopping distance is the total distance a vehicle travels from when the driver sees a hazard to when it stops: stopping distance = thinking distance + braking distance (1 mark). Thinking distance (travelled during the driver's reaction time) increases with higher speed and a longer reaction time (for example from tiredness, alcohol, drugs or distraction) (1-2 marks for two valid factors). Braking distance (travelled while braking) increases with higher speed, and with anything that reduces grip or braking force such as wet or icy roads or worn tyres or brakes (1-2 marks for two valid factors). Markers reward the definition and two correct factors for each part. A common error is to put a braking factor under thinking distance or vice versa.
OCR 20214 marksExplain how a crumple zone reduces the force on the occupants of a car during a collision. Use ideas about momentum and the time taken to stop.Show worked answer →
A P8 question worth four marks on vehicle safety, using momentum (Higher tier ideas). In a collision the car and its occupants undergo a change in momentum as they are brought to rest (1 mark). The force needed depends on how quickly the momentum changes: a force produces the rate of change of momentum, so a larger time to stop means a smaller force (2 marks for force being smaller when the change of momentum happens over a longer time). A crumple zone is designed to crumple and increase the time taken for the car to stop in the crash, so the same change in momentum happens over a longer time, giving a smaller force on the occupants and reducing injury (1 mark). Markers reward the change of momentum, the longer time giving a smaller force, and the crumple zone increasing the collision time. Airbags and seatbelts work the same way.
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