Question

Usha swims in a 90 m long pool. She covers 180 m in one minute by swimming from one end to the other and back along the same straight path. Find the average speed and average velocity of Usha.

Why displacement is taken 0 here?

Answer 1

Length of the pool: 90 m
Average speed:

$Speed=\frac{Distance}{TIme}$

Or,$Speed=\frac{180}{60}$

Or, Speed = 3 m/s

Velocity:
$Velocity=\frac{Displacement}{Time}$

Displacement = 0 ( Since she comes back to the same place)

So,
Average velocity=0
​​​​​​(Displacement is taken zero because, unlike distance, displacement is a vector quantity and it has direction. When Usha swims from one end of the pool to the other end her displacement is 90 m but when she swims back her displacement is -90 m, because she swims back in the opposite direction as a result, total displacement becomes -90 + 90 = 0).