Stopping distance and road safetyIB MYP Physics: Revision notes
Section 1
Stopping distance
When a driver sees danger, the car does not stop at once. The distance it travels has two parts:
stopping distance = thinking distance + braking distance
- Thinking distance: the distance travelled in the driver's reaction time, before the brakes are pressed.
- Braking distance: the distance travelled while the brakes are slowing the car to a stop.
Example: thinking distance 14 m + braking distance 48 m = 62 m.
Section 2
Factors affecting thinking distance
Thinking distance = speed × reaction time. It increases if:
- the speed is higher (twice the speed gives twice the thinking distance)
- the reaction time is longer, because the driver is tired, has drunk alcohol, has taken some drugs, or is distracted (for example using a mobile phone).
The state of the road, tyres and brakes do not change the thinking distance.
Alcohol and tiredness increase thinking distance, not braking distance, because they slow the driver down, not the car's brakes.
Section 3
Factors affecting braking distance
Braking distance increases if:
- the speed is higher
- the road is wet or icy (less friction between tyres and road)
- the tyres are worn (less grip)
- the brakes are worn (a smaller braking force)
- the mass of the vehicle is larger (a heavy lorry takes longer to stop).
The braking force comes from friction. A bigger braking force gives a shorter braking distance.
Section 4
Why speed has such a large effect on braking distance
A moving car has kinetic energy, which depends on mass and speed squared (KE = ½ m v²). The brakes must remove all of it. Work done by the brakes = braking force × braking distance.
If the speed doubles, the kinetic energy becomes four times bigger. With the same braking force, the braking distance becomes four times longer.
Example: braking distance 12 m at 10 m/s, so 4 × 12 = 48 m at 20 m/s.
Thinking distance only doubles at twice the speed, but braking distance quadruples.
Double the speed: thinking distance × 2, braking distance × 4.
Section 5
Seat belts and speed limits
Seat belts (Newton's first law): in a sudden stop the car slows but the passengers keep moving forwards because of their inertia. The belt exerts a backwards force that stops them with the car. It stretches slightly, so the stopping time is longer and the force is smaller.
Speed limits: a lower speed gives a shorter stopping distance and less kinetic energy, so there are fewer collisions and any crash is less severe. Limits are lowest where there are many pedestrians, such as outside schools.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Stopping distance and road safety
- A driver in Cairo is travelling along a straight road when a child runs out in front of her car. There is a short delay before she presses the brake pedal, and then the car travels further before it stops.Explain why drinking alcohol increases the stopping distance of a car.2 marks
- A driving test centre in Auckland tests a car on a dry, level road. At 10 m/s the car has a braking distance of 12 m when the brakes apply a constant force. A learner driver has the same reaction time of 0.8 s whatever her speed.Predict the braking distance of the car at 20 m/s on the same road. Explain your prediction in terms of kinetic energy.2 marks
- Students in Lagos investigate how the road surface affects the braking distance of a model car. The car is released from the same point on a ramp, so it reaches the same speed at the bottom, and then slides to a stop across a flat surface with its wheels locked. The braking distance was 0.90 m on a smooth plastic sheet, 0.50 m on a wooden board, 0.30 m on carpet and 0.20 m on sandpaper.State the independent variable, the dependent variable and one control variable in this investigation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).