Stopping Distance (Oxford AQA IGCSE Physics)

Worked Example

At a speed of 20 m/s, a particular vehicle had a stopping distance of 40 metres. The car travelled 14 metres whilst the driver was reacting to the incident in front of them.

What was the braking distance?

A      54 m

B      34 m

C      26 m

D      6 m

Answer:  C

Step 1: Identify the different variables

• Stopping distance = 40 m

• Thinking distance = 14 m

Step 2: Rearrange the formula for stopping distance

stopping distance = thinking distance + braking distance

braking distance = stopping distance – thinking distance

Step 3: Calculate and identify the correct braking distance

braking distance = 40 – 14

braking distance = 26 metres

Therefore, the answer is C

Worked Example

The graph below shows how the thinking distance of a driver depends on the speed of the car.

(a) Use the graph to describe the relationship between thinking distance and speed.

(b) Some people drive when they are tired, despite warnings against doing so.

Draw a new line on the graph to show how thinking distance varies with speed for a tired driver.

Answer:

Part (a)

Step 1: Check if the line is straight and if it goes through the origin

• The graph shows a straight line through the origin

• Therefore, the thinking distance is directly proportional to the speed of the car

Part (b)

Step 1: Recall the factors which affect the thinking distance

• Three additional factors affect the thinking distance because they affect human reaction time:

  • Tiredness

  • Distractions

  • Intoxication

• Hence, a tired driver's reaction time is greater (i.e. it takes longer for them to react)

Step 2: Draw a line that shows greater thinking distance for the same speed

• At the same speed, a tired driver's thinking distance will be greater than a driver who is alert

• This means a line should be drawn with a steeper gradient, as shown below