The following is a suggested answer for the GCE O Level Physics question in Year 2011 November, Paper 1, multiple choice question 3.

The question provided the speed-time graph of a trolley  as the trolley moves across a floor. The student is required to find the distance travelled by the trolley when it travels at constant speed.

Suggested Answer:

The answer for this mutiple choice question is B. That is, the distance travelled is 12 m.

The distance travelled by the trolley is = area under the speed-time graph when the speed is constant

= (4 m/s) x (3 s)

=12 m


Constant speed means the speed remains the same, unchanged, for  a certain period.

From the speed-time graph provided, the trolley is moving at a constant speed from time=4s to time=7s, i.e. over a duration of 3 seconds. The speed of the trolley is 4 m/s during this constant speed period.


GCE O Level Physics N2011-P1-Q3

Why distance equals area under a Speed-Time Graph?

From the formula :

Speed = Distance travelled/ time

It also means:

Distance travelled= Speed x time

From a Speed-Time graph:

        • the y-axis value gives the speed;
        • the x-axis value gives the time.

In a Speed-Time graph, area under the graph = (y-axis value) x (x-axis value)

= speed x time

= Distance travelled.