P5 - Stopping Distances Flashcards
What is stopping distance equal to?
Stopping Distance = Thinking Distance + Braking Distance
When is stopping distance important?
When you are in an emergency for example there is a hazard ahead, a driver has to perform an emergency stop. This is where the maximum force is applied by breaks in order to stop the car in the shortest possible distance. The longer it takes to perform emergency stop, the higher the risk of crashing into whatever is in front.
What is thinking distance?
How far the car travels during the driver’s reaction time
What is braking distance?
The distance taken to stop under the breaking force (once the brakes are applied)
( Typical car braking distances are: 14 m at 30 mph, 55 m at 60 mph and 75 m at 70 mph)
What is thinking distance affected by?
1) Your SPEED - the faster you’re going the further you’ll travel during the time you take to past
2) Your REACTION TIME - the longer your reaction time, the longer your thinking distance
What is braking distance affected by?
1) Your SPEED - for a given braking force, the fanter a vehicle travels, the longet it takes to stop.
2) 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’s tyres and the road, which can cause tyres to skid.
3) The CONDITION of your IYRES - if the tyres of a vehicle are bad (they don’t have any tread left) then they cannot get rid of water in wet conditions. This leads to them skidding on top of the water
4) 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.
What factors affect stopping distance?
- Icy conditions
- Driving close to other cars ( especially in icy conditions)
- Speed limits
( The longer your stopping distance, the more space you need to leave in front in order to stop safe )
What does braking in a car rely on?
The friction between the brakes and wheels
Why does braking rely on the 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 storeg 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, so the more work needs to be 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 were they may cause brakes to overheat (so they don’t work as well or could cause the vehicle to skid.
A car traveling at a typical speed makes an emergency stop to avoid hitting a hazard
Assume the deceleration is uniform, and
25 m ahead. Estimate the braking force needed to produce this deceleration
Assume the deceleration is uniform, and
25 m ahead. Estimate the braking force needed.
1) rearrange v^2 - u^2 = 2as to find the deceleration.
v = -25 m/s m = -1000 kg.
2) Then use F = ma, with m = -1000 kg
a= (v^2 - u^2) + 2s = (0^2- 25^2)+ (2 × 25) = 12.5 F= 1000 x125 = 12500 N
F = 12500N
