Q 3.25 On a long horizontally moving belt (Fig. ), a child runs to and fro with a speed 9 km h-1  (with respect to the belt) between his father and mother located 50 m apart on the moving belt. The belt moves with a speed of 4 km h-1. For an observer on a stationary platform outside, what is the

(c) time taken by the child in (a) and (b)?

Which of the answers alter if motion is viewed by one of the parents?


Answers (1)
S Sayak

The distance between the parents is s = 50 m

The relative velocity of the child with respect to both his parents remains the same as the parents are also standing on the moving belt.

v = 9 km h-1 = 2.5 m s-1

Time taken by the child in (a) and (b) is t = s/v = 20 s.

As both, the parents are also standing on the belt as well the speed of the child would appear to be 9 km h-1 to both the parents irrespective of the direction in which the child is moving. Therefore answer (a) and (b) would change.

The time taken by an object to travel from one point to another is independent of the observer and therefore answer to question (c) would not change.