The distance time graph for four objects is shown below:
four graphs labeled object a, object b, object c, and object d are shown with distance in miles on y axis and time in hours on x axis. the first graph is a straight slanting line passing through origin, the second graph is a graph consisting of several slanting and horizontal straight lines, the third graph is a curved line passing through the origin and going up, and the last graph is a curved line going up and passing through a point on the y axis
which object is moving at constant speed?

Object A

Explanation:

The speed of an object moving at constant speed is given by:

where

d is the distance travelled

t is the time taken

In a distance-time graph, we see the speed corresponds to the slope of the curve. In fact:

- The variation along the x-axis corresponds to the time interval, t

- The variation along the y-axis corresponds to the variation in position, so the distance travelled, d

So the slope of the curve is

therefore it corresponds to the speed.

This means that the object which is moving at constant speed is the one which has a curve with constant slope: so it's object A, whose curve is a straight line.

. west