Stopping Distances

  • Created by: JJaquiery
  • Created on: 18-02-20 12:09

Stopping Distances

An emergency stop is where maximum force is applied by the brakes in order to stop the car i  the shortest amount of distance as possible

Stopping distance = thinking distance + braking distance

Thinking distance is how far the car travels during the drivers reaction time                       Braking distance is the distance taken to stop under the braking force

Thinking distance is affected by:

  • Your speed (the faster you are going, the further you'll travel in a given time)
  • Your reaction time (the longer the reaction, the greater the thinking distance. This can be affected by tiredness, drugs, alcohol

Stopping distance is affected by:

  • Your speed
  • The weather or road surface (if it is wet or icy, there is less grip, a greater chance of skidding
  • The condition of your tyres (if they lose grip you can skid)
  • Quality of your brakes (can't apply as much force)
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Reaction Times

You can measure reaction times with the ruler drop test:

  • Sit with your arm resting off the edge of a desk and get someone to hold a ruler so it hangs between your thumb and forefinger, lined up with zero
  • Without any warning, the erson holding the ruler shoud drop it, the person being tested should close their thumb and forefinger as quck as possible to try and catch the ruler
  • The measurement now on t e rukr is how far the ruler travelled during the reaction time
  • The longer the distance, the longer the reaction time
  • You can calculate how long the ruler travels for because you kow the average acceleration is 9.8m/s/s, and with this use  v^2- u^2= 2as to calculate the chance is velocity
  • Then use a = change in v / t to work out the time taken, which is your reaction time

Other things to note:

  • You should repeat this process many times
  • Use the same ruler and same person dropping the ruler during the repeats
  • You can calculate the mean time after many repetitions
  • Other factors can be introduced, such as distractions, to investigate the effects of them by comparing the times with the original reaction times
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Braking Distances

When a brake pedal is pushed, this causes brake pads to be ushed against the wheel, which causes friction and ,consequently, work done.

The work done between the brakes and the wheels transfers energy form the kinetic energy store of the wheels to the thermla energy stores of the brakes, so the brakes increase in temperature

The faster a vehicle is oing, the more energy is has in its kinetic energy store, so  more work has to be done to stop it. This means a reater braking force is needed to stop it within a certain distance

A larger braking force means a larger deceleration, very large decelerations can be dangerous as they can cause the brake to overheat (so they don't work as well) or cause the vehicle to skid

To estimate the braking force needed to stop a car within a certain distance, use v^2 - u^2 = 2as to work out the deceleration and f = ma to work out the total force needed to stop the vehicle

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Speed an stopping distances

To avoid an accident, drivers need to leave a ceratin amount of space between them and the car in front of them, to allow eniugh space to stop

Speed limits are extremely important because sped affects the stopping distance so much

As a cars speed increases, the thinking distance increases at the same rate, the graph is linear. This is because thinking distance stays pretty constant, but the higher the speed, the more distnce you travel in that time

Howver, braking distance increases faster the more you speed up, this is because the work done needed to stop the car is equal to the energy in its kinetic store. As seed doubles, the kinetic energy store increases 4-fold, so the work done to stop the car also increases 4-fold

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