On a long horizontally moving belt, 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, the speed of the child running in the direction of motion of the belt is

Explanation

(a) Speed of the child running in the direction of motion of the belt `=9+4 =13 kmhr^(-1)`
(b) Speed of the child in opposite direction of the motion of the belt `=9-4=5 kmhr^(-1)`
(c) Since both the parents and child are on the moving belt. So their relative position and speed remain unchanged.
speed of child `=9xx(5)/(18)=2.5 ms^(-1)`, time taken `=("distance")/("speed")=(50)/(2.5)=20s`
If the motion is =vieqed by one of the parents answer to (a) and (b) are altered while answer to (c) remain unchanged.