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Time and Speed

A boy walks f...

Question

A boy walks from his classroom to the bookshop along a straight corridor towards North. He covers a distance of 20 m is 25 seconds to reach the bookshop. After buying a book, he travels the same distance at the same time to reach back in the classroom. Find (a) average speed, and (b) average velocity, of the boy.

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Solution

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
Distance from classroom to bookshop = 20m
Time = 25sec
After buying a book, he travels the same distance at the same time to reach back into the classroom.
(a) Average speed = $\frac{T o t a l D i s t a n c e}{T o t a l T i m e}$ = $\frac{20 + 20}{25 + 25}$ = $\frac{40}{50}$ = 0.8 m/s
(b) Average velocity= $\frac{D i s p l a c e m e n t}{T i m e}$
As displacement is the shortest path between source and destination.
Here, displacement = 0, because child travels 20m forward and travels 20 m backward.
Displacement = 20 - 20 = 0
Hence, average velocity = $\frac{0}{50}$ = 0 m/s

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