Stopping Distances

Many factors affect your total stopping distance

  • In an emergency, a driver may perform an emergency stop. This is where maximum force is applied by the brakes in order to stop the car in the shortest possible distance. The longer it takes to perform an emergency stop, the higher the risk of crashing into whatever’s in front.
  • The distance it takes to stop a car in an emergency is found by:

  Stopping distance = Thinking Distance + Braking Distance

  • Where the THINKING DISTANCE is how fast the car travels during the driver’s reaction time. And the BRAKING DISTANCE is the distance taken to stop under the braking force. Typical car barking distances are: 14m at 30mph, 55mm at 60mph and 75m at 70mph
  • Thinking distance is affected by:
  • Your SPEED-the faster you’re going the further you’ll travel during the time you take to react
  • Your REACTION TIME-the longer your reaction time the longer your thinking distance

  Braking distance is affected by:

  • Your Speed-fora given braking force, the faster a vehicle travels, the longer it takes to stop

  • The WEATHER or ROAD SURFACE-if it is wet or icy, or there are leaves or oil on the road, there is less grip, and so less friction, between a vehicle’’ tyres an the road, which can cause tyres to skid

  • The CONDITION of your TYRES-if the tyres of a vehicle are bald, then they cannot get rid of water in wet conditions. This leads to them skidding on top of the water.

  • How good your BRAKES are-if brakes are worn or faulty, they won’t be able to apply as much force as well-maintained brakes, which could be dangerous when you need to brake hard

  • You need to be able to describe the factors affecting stopping distance and how this affects safety=especially in an emergency

  • Speed limits are really important because speed affects the stopping distance

Braking relies on friction between the brakes and wheels

  • When the brake pedal is pushed, this causes brake pads to be pressed onto the wheels. This contact causes friction, which causes work to be done, The work done between the brakes and the wheels transfers energy from the kinetic energy stores of the wheels to the thermal energy stores of the brakes. The brakes increase in temperature
  • The faster a vehicle is going, the more energy it has in its kinetic stores, s the more work needs to done to stop it. This means that a greater braking force is needed to make it stop within a certain distance
  • A larger braking force means a larger deceleration. Very large decelerations can be dangerous because they may cause brakes to overheat or could cause the vehicle to skid
  • You can estimate the forces involved in accelerations of vehicles using typical values:
  • Assume the deceleration is uniform, and rearrange v2-u2-2as to find the deceleration
  • Then use F=ma with m=1000kg
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