A lorry travels at a constant speed and then decelerates until it stops.

Question 28

A lorry travels at a constant speed and then decelerates until it stops.

Which graph shows the variation with time of the distance travelled by the lorry?

Reference: Past Exam Paper – November 2016 Paper 12 Q7

Solution:

Answer: B.

The gradient of a distance-time graph gives the speed of the lorry.

The lorry first travels at constant speed. So, the gradient should be constant at first. That is, it should be a straight line. [A incorrect]

A horizontal line means that the gradient and hence, the speed is zero. This cannot be correct as the lorry IS moving at constant speed. [C incorrect]

The lorry then decelerates until it stops. That is, its speed decreases until it becomes zero. In the graph, the gradient should decrease until it becomes a horizontal line (speed = 0). [B is correct]

Recall that the gradient of the distance-time graph gives the speed. Speed is a scalar quantity, and it makes no sense to have a negative speed as indicated by the gradient of graph D. [D is incorrect]

Also, as the lorry moves, the distance travelled (a scalar quantity) should increase. It cannot decrease since this is not ‘displacement’.