On a long horizontally moving belt, a child runs to an fro with a speed `9 kmh^(-1)` ( with respect to the belt) between his father and mother located 50m a part on the moving belt. The belt moves with a speed of 4 `kmh^(-1)`. For an observer on a stationery platform outside, what is the
(i) speed of the child running in the direction of motion of the belt,
(ii) speed of the child running opposited to the direction of motion of the belt , and (iii) time taken by child in (i) and (ii) ?
which of the answers alter if motion is viewed by one of the parents?

Options

A. `4 km h^(-1)`
B. `5 km h^(-1)`
C. `9 km h^(-1)`
D. `13 km h^(-1)`

Answer

D

Explanation

Figure shows conditions of the question.
In this case,
Speed of belt w.r.t ground `therefore` `v_(BG) = 4 km h^(-1)`
Speed of child w.r.t belt `therefore` `v_(CB) = 9 km h^(-1)`
`therefore` For an observer on a stationary platform, speed of child running in the direction of motion of the belt is
`v_(CG) = v_(CB) + v_(BG) = 9 km h^(-1) + 4 km h^(-1) = 13 km h^(-1)`