P2.1.3 - AQA GCSE Physics Unit 2

 

 

P2.1.3

Forces and braking

a) When a vehicle travels at a steady speed the resistive forces balance the driving force.

See here.

Candidates should realise that most of the resistive forces are caused by air resistance.

 

b) The greater the speed of a vehicle the greater the braking force needed to stop it in a certain distance.

Candidates should understand that for a given braking force the greater the speed, the greater the stopping distance.

c) The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver's reaction time (thinking distance) and the distance it travels under the braking force (braking distance).

d) A driver's reaction time can be affected by tiredness, drugs and alcohol.

Candidates should appreciate that distractions may affect a driver's ability to react.

e) When the brakes of a vehicle are applied, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increase.

KE is transferred to heat energy as work is done by the force of friction.

 

f) A vehicle's braking distance can be affected by adverse road and weather conditions and poor condition of the vehicle.

Candidates should understand that 'adverse road conditions' includes wet or icy conditions. Poor condition of the car is limited to the car's brakes or tyres.

By saying 'limited to' here, the examiner means you won't get marks for mentioning anything else - even if you make a valid point!