Forces
stopping distance
the stopping distance of a vehicle is thinking distance + braking distance
thinking distance - the distance the vehicle travels during the drivers reaction time
braking distance - the distance it travels under the braking force
the greater the speed the greater the stopping distance
typical car speeds:
- 30mph - 14m
- 60mph - 55m
- 70mph - 75m
reaction time
typical values range from 0.2s - 0.9s
a drivers reaction time can be affected by tiredness, drugs, alcohol and distractions
braking distance
can be affected by weather conditions (wet or icy) and poor condition of the vehicle (brakes or tyres)
when a force is applied to the brakes of a vehicle, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temp of the brakes increases
the greater the speed the greater the braking force needed to stop the vehicle in a certain distance
the greater the braking force the greater the deceleration
large decelerations can lead to brakes overheating or loss of control
conservation of momentum
conservation of momentum - in a closed system the total momentum before an event is equal to the total momentum after the event
cars
crumple zones - crumple on impact increasing the time taken for the car to stop
seat belts - stretch slightly increasing the time taken for the wearer to stop
air bags - inflate before you hit the dashboard of a car and the compressing air inside slows you down more gradually
cars are designed to slow people down because the longer it takes for a change in momentum the smaller the rate of change in momentum so the smaller the force
bike helmets - contain a crushable layer of foam which helps to lengthen the time taken for your head to stop in a crash which reduce the impact on your brain
crash mats - increase the time taken fro you to stop if you fall of them as they are made from soft compressible materials
when force acts on an object that is moving a change in momentum occurs