Question

On a long horizontally moving belt, a child runs to and fro with a speed $9km{h}^{-1}$ (with respect to the belt) between his father and mother located $50m$ apart on the moving belt. The belt moves with a speed of $4km{h}^{-1}$ . For an observer on a stationary platform, the speed of the child running in the direction of motion of the belt is

• A. $4km{h}^{-1}$
• B. $5km{h}^{-1}$
• C. $9km{h}^{-1}$
• D. $13km{h}^{-1}$

Answer 1

Answer: (D)

Figure shows conditions of the question.

In this case,
Speed of belt w.r.t. ground
$\therefore v_{BG}=4km{h}^{-1}$
Speed of child w.r.t. belt
$\therefore v_{CB}=9km{h}^{-1}$
$\therefore$ For an observer on a stationary platform, speed of child running in the direction of motion of the belt is
$v_{CG}=v_{CB}+v_{BG}=9km{h}^{-1}+4km{h}^{-1}$
$=13km{h}^{-1}$