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On a long horizontally moving belt figure, 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 outside, what is the (a) speed of the child running in the direction of motion of the belt ?. (b) speed of the child running opposite to the direction of motion of the belt? (c) time taken by the child in (a) and (b)? Which of the answers alter if motion is viewed by one of the parents?
On a long horizontally moving belt figure, 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 outside, what is the (a) speed of the child running in the direction of motion of the belt ?. (b) speed of the child running opposite to the direction of motion of the belt? (c) time taken by the child in (a) and (b)? Which of the answers alter if motion is viewed by one of the parents?
CBSE | Class 11 | Motion in a Straight Line | NCERT | Physics
On a long horizontally moving belt figure, 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 outside, what is the (a) speed of the child running in the direction of motion of the belt ?. (b) speed of the child running opposite to the direction of motion of the belt? (c) time taken by the child in (a) and (b)? Which of the answers alter if motion is viewed by one of the parents?

Answer –

According to the question, speed of child is 9 km h-1 speed of the belt is 4 km h-1
(a) When the boy runs in the belt’s direction of motion, his speed as measured by a stationary observer equals :

= (9 + 4) km h-1

= 13 km h-1.

(b) When the boy runs in the opposite direction of the belt’s motion, the child’s speed as measured by a stationary observer

= (9 – 4) km h-1

= 5 km h-1

(c) And the distance between the two parents is 50 m, or  0.05 km

We know that the speed of the boy for both the parents is 9 km h-1

Therefore, the time taken by the boy to move in direction of one of the parents is

=0.05 km/9k h-1

=0.0056 h

=20 seconds

i

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