Question

A boy walks from his classroom to the bookshop along a straight corridor towards North. He covers a distance of 20 m in 25 seconds to reach the bookshop. After buying a book, he travels the same distance in the same time to reach back in the classroom.

Find (a) average speed and (b) average velocity of the boy.

Answer 1

(a) The average speed of the boy is given by the ratio of total distance travelled by the boy and the total time taken to cover it. During the complete trip from classroom to bookshop and bookshop to the classroom, the boy travels a total distance of 40 m in 50 seconds of time.

So, Average speed = $\frac{\mathrm{Total}\mathrm{distance}}{\mathrm{Total}\mathrm{time}\mathrm{taken}}=\frac{40\mathrm{m}}{50\mathrm{s}}=0.8\mathrm{m}/\mathrm{s}$

(b) The average velocity of the boy is given by the ratio of total displacement and the total time taken by the boy. The total displacement of the boy during the complete trip of 50 seconds from classroom to bookshop and bookshop to the classroom is zero.

So, Average Velocity = $\frac{\mathrm{Total}\mathrm{displacement}}{\mathrm{Total}\mathrm{time}\mathrm{taken}}=\frac{0}{50\mathrm{s}}=0$