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.
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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.
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