# Physics - Mechanics - Forces, vehicles and safety

Detailed notes that will help when revising for mechanics

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Chapter 6: Forces, vehicles and safety
Stopping safely
Stopping distance is made up of two parts: the thinking distance and the braking distance
Stopping distance = thinking distance + braking distance
Thinking distance is directly proportional to the speed of the car
Braking distance is directly proportional to the square of the speed
Factors affecting stopping distance
Drugs and alcohol will affect the thinking distance of the driver as they take longer to react
Older people also tend to have slower reactions
Factors affecting braking distance
Poorly maintained car
Faulty brakes or tyres (no tread depth)
Road conditions
Weather conditions
Mass of the car
Speed of the car
Car safety features
Passenger cells, crumple zones and engines
The passenger cell is a safety cage where passengers are protected
The rigid cell is more likely to survive an impact
The crumple zone crumples on impact, absorbing a lot of energy
The engine is forced downwards to avoid being directed at the passenger cell
Seat belts
Seat belts help keep passengers in their seat, to avoid collisions with car parts
The seat belt will automatically lock when pulled to fast (hence keeping the passenger in the
seat)
The seat belt is slightly stretchy so the passenger isn't brought to an immediate halt (which
could be very damaging towards bones and tissue)
Air bags
If the car suddenly decelerates, the bag inflates and the person is cushioned
Holes in the air bag means that the bag immediately deflates to avoid the person
rebounding
Controlling impact forces
All of these methods of controlling the impact increase the time of impact
This reduces the acceleration of the collision
So causes the force to be reduced as well (F = ma)
Force and acceleration are directly proportional hence the result above
GPS navigation ­ how it works

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GPS (Global positioning system)
GPS satellites send out regular signals with information about their identity and the time of
transmission
The receiver compares these signals and works out the time lag
This is used to work out the time taken for the signal to travel from the satellite to the receiver
Because we know the speed of the signal we can now work out the distance from each of the
satellites
This is repeated for three satellites and so the position of each of the…read more

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