Question

Which of the following is an example of a motion in which the average speed is not zero, but the average velocity is zero?

• A. A runner runs $200m$ on a straight track of length $100m$
• B. A runner runs $150m$ on a straight track of length $100m$
• C. A runner runs $175m$ on a track of length $100m$
• D. A runner runs $205m$ on a track of length $100m$

Answer 1

The correct option is A A runner runs $200m$ on a straight track of length $100m$

$\text{Average Speed}=\frac{\text{Total Distance}}{\text{Total Time}}$

$\text{Average Velocity}=\frac{\text{Total Displacement}}{\text{Total Time}}$

Average velocity is zero when total displacement is zero and total displacement is zero when the runner returns to the starting point.
The track length is $100m$.So, if he runs $200m$, he goes and comes back to the starting point. So his displacement is zero and hence the average velocity is zero. But since he has covered a distance of $200m$, his average speed is not zero.