Stopping Distances Flashcards

All 30 cards in this deck

What is the thinking distance of a vehicle?

The distance the vehicle travels during the driver's reaction time, before the brakes are applied.

What is the braking distance of a vehicle?

The distance the vehicle travels under the braking force, from the brakes being applied until it stops.

True or false? Thinking distance is the distance travelled while the brakes are being applied.

False. That is the braking distance; thinking distance is travelled during the driver's reaction time, before braking starts.

What are the steps to calculate thinking distance?
e.g. a car travels at $20$ m/s and the driver's reaction time is $0.5$ s

1. Use thinking distance $=$ speed $\times$ reaction time
2. $20 \times 0.5 = 10$ m

True or false? Increasing the speed of a vehicle increases both its thinking distance and its braking distance.

True. A faster vehicle travels further during the reaction time and needs a longer distance to stop for a given braking force.

What is the stopping distance of a vehicle?

Stopping distance $=$ thinking distance $+$ braking distance.

For a given braking force, how does stopping distance change as the speed of a vehicle increases?

The greater the speed, the greater the stopping distance.

What are the steps to find stopping distance from a graph of thinking distance and braking distance against speed?
e.g. at $80$ km/h thinking distance is $15$ m and braking distance is $38$ m

1. Read the thinking distance at that speed: $15$ m
2. Read the braking distance at that speed: $38$ m
3. Add them: $15 + 38 = 53$ m

True or false? Doubling the speed of a car doubles its braking distance.

False. Braking distance increases much more than twofold (roughly with speed squared); it is the thinking distance that doubles.

Roughly how does a car's emergency stopping distance at $60$ mph compare with that at $30$ mph?

About three times greater (roughly $73$ m compared with $23$ m), because braking distance rises much faster than speed.

What is the typical range of human reaction times?

About $0.2$ s to $0.9$ s.

What are the steps in the ruler drop test for measuring reaction time?
e.g. the ruler falls $20$ cm before it is caught

1. One person holds a ruler vertically with the zero mark between the other person's open finger and thumb.
2. The ruler is released without warning and caught as quickly as possible.
3. Measure the distance fallen and convert it to a time using a conversion table or equation: $20$ cm $\approx 0.2$ s.

In the ruler drop test, what does a longer drop distance before the ruler is caught tell you?

The person has a longer reaction time.

How can the ruler drop test be made more reliable?

Repeat the test several times and calculate a mean reaction time.

True or false? All people have the same reaction time.

False. Reaction times vary from person to person, typically between $0.2$ s and $0.9$ s.

Name three things that can increase a driver's reaction time.

Tiredness, drugs and alcohol (distractions also affect the ability to react).

Give two examples of distractions that could increase a driver's thinking distance.

Using a mobile phone and talking to passengers (also loud music).

Apart from reaction time, what factor affects a vehicle's thinking distance?

The speed of the vehicle, because thinking distance $=$ speed $\times$ reaction time.

True or false? Drinking alcohol increases a driver's braking distance.

False. Alcohol increases reaction time, so it increases the thinking distance (and therefore the stopping distance).

True or false? If data show that mean reaction time increases after drinking alcohol, then every person's reaction time must have increased.

False. A mean can increase even though the change is not the same for all people.

Why does an icy or wet road increase braking distance?

There is less friction (grip) between the tyres and the road, so the braking force is smaller.

Why do worn brakes or worn tyres increase braking distance?

They produce less friction, so the braking force is smaller and the car travels further before stopping.

True or false? Tiredness increases the braking distance of a car.

False. Tiredness increases reaction time, which increases thinking distance, not braking distance.

True or false? In wet or icy conditions drivers should leave a bigger gap to the vehicle in front.

True. Braking distance is greater in adverse conditions, so a longer stopping distance is needed.

What energy transfer happens when the brakes of a vehicle are applied?

Work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and increases the internal (thermal) energy, so the brakes' temperature rises.

What are the dangers of a very large deceleration when braking?

The brakes may overheat and/or the driver may lose control of the vehicle (skidding).

To stop a vehicle within a certain distance, how does the braking force needed depend on the vehicle's speed?

The greater the speed, the greater the braking force needed.

What are the steps to estimate the braking force on a car using energy?
e.g. a $1000$ kg car travelling at $10$ m/s stops in $25$ m

1. Find the kinetic energy: $\frac{1}{2} \times 1000 \times 10^2 = 50\,000$ J
2. Work done by the brakes equals this energy: $F \times d = 50\,000$ J
3. $F = 50\,000 \div 25 = 2000$ N

What are the steps to estimate the braking force from a vehicle's deceleration?
e.g. a $1200$ kg car decelerates at $5$ m/s$^2$

1. Use $F = m\,a$
2. $F = 1200 \times 5 = 6000$ N (acting backwards on the car)

True or false? A greater braking force produces a greater deceleration of the vehicle.

True. By $F = m\,a$, for a given mass a larger braking force gives a larger deceleration.