Question

On a long horizontally moving belt (fig.) a child runs to and fro with a speed 9 kmh^-1 (w.r.t the belt) between his father and mother located 50 m apart on the moving belt. The belt moves with a speed of 4 km^-1. For an observer on a statinary 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 1

vector vector v_B = velocity of belt = 4 kmh^-1(left to right)

vector v_CB = velocity of the child with respect to the belt.

(a) When child runs in the direction of motion of the belt:

Velocity of the child w.r.t belt = 9 kmh^-1 (from left to right). If velocity of the child w.r.t. an observer on the platfrom is vector v_c, then 

(b) When the child runs opposite to the direction of motion of the belt:

Now vector v_CB = -9kmh^-1   (from left to right)

If velocity of the child w.r.t stationary observer is vector v_C3,then

Negative sign indicates opposite direction of the motion of the child i.e., from right to left.

(c) The taken in conditions (a) or (b)

t = Distance between parents/Speed of the child

= 50 x 60 x 60/1000 x 9 = 20s.

Time remains 20s only if one of the parents view the child.