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Question: On a long horizontally moving belt, a child runs to and fro with a speed 9 km h<sup>-1</sup> (with r...

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

A

4 km h-1

B

5 km h-1

C

9 km h-1

D

13 km h-1

Answer

13 km h-1

Explanation

Solution

Figure shows conditions of the questions .

In this case, speed of belt w.r.t ground VBG=4kgh−1V_{BG} = 4kgh^{- 1}

Speed of child w.r.t. belt VBG=9kgh−1V_{BG} = 9kgh^{- 1}

∴\thereforeFor an observer on a stationary platform, speed of child running in the directions of motion of the belt is VCG=VCB+VBG=9kmh−1+4kmh−1=13kmh−1V_{CG} = V_{CB} + V_{BG} = 9kmh^{- 1} + 4kmh^{- 1} = 13kmh^{- 1}