Question

A particle covers a distance from $A$ to $B$ over a period of time; the distance versus time plot is the shown below. Then which of the following is true for the motion of the particle?

• A. Both average speed and instantaneous speed are always zero.
• B. Average speed is always non-zero but instantaneous speed can be zero.
• C. Instantaneous speed is always non-zero but average speed can be zero.
• D. Both average speed and instantaneous speed are always non-zero.

Answer 1

Answer: (B)

The distance versus time plot for movement of a particle from $A$ and $B$ is shown in the diagram.

From graph, it is a slope of distance-time graph for position $A$ to $C$ and $D$ to $B$ is always positive, therefore both the average speed and instantaneous speed are not always zero, therefore, option (a) is not correct. Average velocity
$=\frac{\text{ Total displacement from }A\text{ to }B}{\text{ Total time interval }}$
Since, total displacement depends on straight distance between initial and final points, hence, displacement between $A$ to $B$ always non zero, therefore the average speed is always non zero. Since, no distance is travelled between point $C$ to $D$ is zero. Hence, instantaneous speed at every instant between $C$ to $D$ is zero. Hence, option (b) is correct and option (c) and (d) are incorrect.