Question

On a long horizontally moving belt, a child runs to and from 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 opposite to 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 1

Answer: (B)

5 km h^-1

Answer 2

In this case

$V_{BG} = 4kmh^{- 1}$

$V_{BG} = 9kgh^{- 1}$

$\therefore$For an observer on a stationary platform, speed of child running opposite to the directions of motions of the belt is $V_{CG} = V_{CB} + V_{BG} = - 9kmh^{- 1} = - 5kh^{- 1}$

- ve sing shows that the child appears to be moving from right to left with a speed of $5kmh^{- 1}.$