The motion of an object can be predicted by analysing the forces that act on the object. Balanced forces have no effect, while unbalanced forces can lead to change in acceleration.

This is what a velocity-time graph represents. It helps you calculate reaction time and the time it takes for a vehicle to stop.

Look at the graph and answer the questions.

- Question
What is the driver’s reaction time?

0.8 s

- Question
What was the speed of the vehicle before it started to brake?

15 m/s

- Question
What distance was travelled while the driver was reacting (the thinking distance)?

The area under the flat part of the graph, which is 15 m/s × 0.8 s = 12 m.

- Question
How long did it take the car to stop?

The time between 0.8 s and 4 s = 3.2 s.

- Question
What is the braking distance?

The area of the triangle.

½ × base × height = ½ × 3.2 s × 15 m/s = 24 m

- Question
What is the total stopping distance?

12 m + 24 m = 36 m