Stopping Distances Flashcards
All 24 cards in this deck
What is the stopping distance of a vehicle made up of?
Stopping distance = thinking distance + braking distance
What is meant by the thinking distance of a vehicle?
The distance travelled during the driver's reaction time, before the brakes are applied.
What is meant by the braking distance of a vehicle?
The distance travelled from the brakes being applied until the vehicle stops.
What happens to a car's kinetic energy when it brakes?
It is transferred to thermal energy of the brake pads, wheels and the surroundings.
True or false? The stopping distance is the distance a car travels once the brakes have been applied.
False. That is the braking distance; stopping distance also includes the thinking distance.
True or false? The kinetic energy of a braking car is destroyed as it slows down.
False. Energy is never destroyed — it is transferred to thermal energy in the brakes and surroundings.
What are the steps to measure reaction time using the ruler drop test?
e.g. the ruler is caught after falling 20 cm
- A partner holds a ruler vertically with the zero mark between your open fingers.
- They drop it without warning; catch it as quickly as you can and read the mark at your fingers: 20 cm.
- Convert the distance to a time using a conversion chart: about 0.2 s.
What is a typical human reaction time?
About 0.2 s to 0.9 s.
In the ruler drop test, what does a greater distance fallen before the ruler is caught tell you?
The person has a longer reaction time.
Name a method of measuring reaction time other than the ruler drop test.
A computer-based test: click the mouse (or press a key) as soon as the screen changes.
Why is a computer reaction-time test better than the ruler drop test?
The computer times the response automatically, so there is no human timing error and it measures to the nearest millisecond.
True or false? Repeating a reaction time test several times and taking a mean makes the result more reliable.
True. Reaction times vary, so a mean of repeats reduces the effect of random variation.
Which factors affect the thinking distance of a vehicle?
The speed of the vehicle and the driver's reaction time.
Which factors affect the braking distance of a vehicle?
Speed, mass of the vehicle, state of the brakes, state of the road, and the friction between the tyres and the road.
Which factors increase a driver's reaction time?
Drugs, alcohol, tiredness and distractions such as a mobile phone or loud music.
How does a greater vehicle mass affect the stopping distance?
It increases the braking distance — there is more kinetic energy to remove for the same braking force.
True or false? Worn brakes and a wet road increase the thinking distance.
False. They increase the braking distance; thinking distance depends only on speed and reaction time.
True or false? An icy road increases the braking distance because there is less friction between the tyres and the road.
True. Less friction means a smaller braking force, so the car travels further before stopping.
What is the equation for the work done by the brakes in bringing a vehicle to rest?
work done = braking force × braking distance
What are the steps to find a car's braking distance from its kinetic energy?
e.g. kinetic energy J, braking force N
- Work done by the brakes = initial kinetic energy: J.
- Rearrange work done = braking force × braking distance: .
- m.
How does braking distance depend on the initial speed of a vehicle?
Braking distance is proportional to speed squared (), so doubling the speed makes it four times as long.
True or false? Doubling a car's speed doubles both the thinking distance and the braking distance.
False. Thinking distance doubles, but braking distance becomes four times as large.
Roughly what is the stopping distance of a car travelling at 30 mph (about 13 m/s) in good conditions?
About 23 m (roughly 9 m thinking + 14 m braking).
What shape is a graph of braking distance against initial speed for a car?
An upward curve through the origin, because .