car safety

• stopping distance is the sum of...

  two distances
• stopping distance =


  thinking distance + braking distance
• thinking distance


  how far the car travels during the driver's reaction time (the time between the driver seeing a hazard and applying the breaks)
• braking distance


  the distance travelled before stopping under the braking force (once the brakes are applied)
• what energy transfer occurs when a car brakes?

  energy is transferred from the kinetic energy store of the wheels to the thermal energy store of the brakes
• thinking distance is affected by...

  - speed: the faster you're going the further you'll travel during the time you take to react

  - reaction time: the longer your reaction time, the longer your thinking distance
• reaction time is affected by...

  tiredness, drugs, alcohol, distractions
• reaction time required practical

  apparatus: metre ruler and chair or stool

  1) use your non dominant hand

  2) sit on a chair or stool and place your non dominant hand out in front of you

  3) your partner will stand and hold a ruler vertically with the bottom end(0cm) in between your thumb and first finger

  4) your partner should drop the ruler without telling you and you should catch the ruler as quickly as you can

  5) repeat but this time your partner should have a caffeinated drink (to speed up reaction time) or with distractions (to slow down reaction time)

  control variables: person catching the ruler, volume of caffeinated drink, ruler
• braking distance is affected by...

  - speed: for a given braking force, the faster a vehicle travels, the longer it take to stop

  - the weather and road surface: if it is wet or icy, or there are leaves or oil on the road, there is less friction between a vehicle's tyres and the road

  - tyre/brake condition: if the tyres/brakes are worn, they will be less effective

  - vehicle mass/people inside the vehicle: a heavier vehicle will stop slower due to having more momentum
• the longer the stopping distance...


  the more space you need to leave in front of the car in order to stop safely
• why are speed limits important?

  because speed greatly affects the stopping distance, even more than thinking distance
• larger braking distance means a ... deceleration


  larger
• the faster a vehicle is going, the more energy it has in its...

  kinetic store, so more work needs to be done to stop it
• braking distance is ... to the square of initial speed

  proportional

  (as speed doubled, the work done needed to stop the car quadruples (2²)
• why do cars have safety features?

  they absorb kinetic energy transferred by collision, reducing injury to people in the car. they also increase the time take for the change in momentum to happen, reducing the forces involved
• crumple zone


  the region of a vehicle designed to absorb the energy of impact by increasing the time of the impact
• bumpers


  these absorb the impact during a crash
• air bags


  these inflate before you hit the dash board of a car, the compressing air inside it slows you down more gradually than if you had just hit the hard dashboard
• seat belts


  these stretch slightly, increasing the time taken for the wearer to stop
• helmets


  have a crushable layer of foam which helps lengthen the time taken for your head to stop in a crash, reducing the impact on your brain
• crash mats and cushioned playground flooring

  increase the time taken for you to stop if you fall on them. this is because they are made from soft, compressible materials
• many safety features decrease the rate of...

  momentum