Safety in Public TransportOxford AQA IGCSE Physics: Revision notes
Section 1
What forces act on a vehicle travelling at steady speed?
When a vehicle travels at a steady speed in a straight line, the resultant force on it is zero — this means the resistive forces (friction, air resistance) acting backwards are exactly balanced by the driving force pushing it forwards.
If the driving force increases above the resistive forces, the vehicle accelerates; if the resistive forces exceed the driving force (e.g. brakes applied), the vehicle decelerates.
State explicitly that at constant velocity, resultant force = zero, not just that the vehicle 'isn't speeding up'.
Section 2
Why does braking need to be considered carefully?
The faster a vehicle is travelling, the greater the braking force needed to stop it within a given distance.
- Large decelerations require very large braking forces
- This can cause brakes to overheat, reducing their effectiveness
- It can also cause a driver to lose control of the vehicle, especially at high speed
This is why speed limits and safe braking practices matter for public transport safety.
Don't just say 'braking is dangerous' — link the physics: higher speed needs greater braking force, which risks brake overheating or loss of control.
Section 3
What is stopping distance made up of?
Stopping distance = thinking distance + braking distance
- Thinking distance: the distance travelled during the driver's reaction time, before the brakes are applied
- Braking distance: the distance travelled once the brakes are applied, until the vehicle stops
Factors that increase thinking distance (longer reaction time):
- Tiredness
- Distractions (e.g. mobile phones)
- Drugs
- Alcohol
Always separate causes of increased thinking distance (driver factors: tiredness, drugs, alcohol, distraction) from causes of increased braking distance (road/vehicle factors).
Section 4
What energy transfer happens when braking?
When brakes are applied, friction between the brakes and the wheels converts the vehicle's kinetic energy into thermal energy (heat). This is why brakes get hot after repeated or heavy use.
The greater the vehicle's speed (and therefore kinetic energy), the more thermal energy must be transferred to stop it — which is part of why stopping a fast-moving vehicle requires a much greater braking distance.
Section 5
What increases braking distance?
Braking distance is increased by:
- Adverse road or weather conditions: wet, icy or loose surfaces reduce the grip (friction) between the tyres and the road
- Poor vehicle condition: worn or faulty brakes reduce the braking force available; worn or under-inflated tyres reduce grip
All of these mean a greater distance is needed to bring the vehicle to a stop safely.
A bus with worn tyres travelling on a wet road will need a much longer braking distance than the same bus with good tyres on a dry road.
Must Know
- At steady speed in a straight line, resistive forces balance the driving force (resultant force = 0)
- Greater speed requires greater braking force to stop in a given distance
- Large decelerations can overheat brakes or cause loss of control
- Stopping distance = thinking distance + braking distance
- Thinking distance increases with tiredness, distraction, drugs and alcohol
- Braking converts kinetic energy to thermal energy via friction
- Adverse weather/road conditions and poor vehicle condition (brakes, tyres) increase braking distance
That's the notes covered.
Carry on to the next subtopic.