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 speedtime 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 speedtime graph when the speed is constant
= (4 m/s) x (3 s)
=12 m
Explanation:
Constant speed means the speed remains the same, unchanged, for a certain period.
From the speedtime 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.
Why distance equals area under a SpeedTime Graph?
From the formula :
Speed = Distance travelled/ time
It also means:
Distance travelled= Speed x time
From a SpeedTime graph:



 the yaxis value gives the speed;
 the xaxis value gives the time.


In a SpeedTime graph, area under the graph = (yaxis value) x (xaxis value)
= speed x time
= Distance travelled.