Answer :

Given,

Relative speed of child with respect to belt, v_{bc} = 9 km/h

Speed of the belt, v = 4 km/h

Distance between parents, d = 50 m

(a) Speed of the child for stationary observer when he moves in same direction of belt is given by, V = v_{bc} + v

= 9 + 4 = 13 km/h

(b) Speed of the child for stationary observer when he moves in opposite direction of belt is given by, V = v_{bc} – v

= 9 – 4 = 5 km/h

(c) Since, parents are stationary with respect to belt, child speed is same for both. That is v_{bc} = 9 km/h = 2.5 m/s

⇒ time taken by child to move towards one of his parents, t =

∴ t = = 20 s

If the motion is viewed in parent’s perspective, answer in (a) and (b) will alter and answer in (C) unaltered. Because, for them child speed is constant and equal to 9 km/h.

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