NCERT Solutions
Class 11 Physics
Motion In A Straight Line

Q. 2.9
Two towns A and B are connected by a regular bus service with a bus leaving in either direction every T minutes. A man cycling with a speed of 20 km h–1 in the direction A to B notices that a bus goes past him every 18 min in the direction of his motion, and every 6 min in the opposite direction. What is the period T of the bus service and with what speed (assumed constant) do the buses ply on the road?
Let V be the speed of the bus running between towns A and B.
Speed of the cyclist, vc = 20 km/h
Case 1:- Relative speed of the bus moving in the direction of the cyclist
= (V – vc) = (V – 20) km/h
Since the bus goes past the cyclist every 18 min i.e., (18/60),
Therefore, the distance covered by the bus w.r.t the cyclist
= (V – 20) x (18/60) km (i)
As the bus leaves after every T minutes, the distance travelled by the bus will be
= (V x (T/60)) (ii)
As both equations (i) and (ii) are equal.
= (V – 20) x (18/60) = (VT)/ (60) (iii)
Case 2:- Relative speed of the bus coming from town q to p w.r.t cyclist
= (V+20) kmh-1
Time taken by the bus to go past cyclist = 6 min = (6/60) h
Therefore (V +20) (6/60) = (VT)/ (60) (iv)
From equations (iii) and (IV)
(V+20) (6/60) = (V – 20) x (18/60)
(V + 20) = 3V – 60
2V = 80
V = 40km/h
Substituting the value of V in equation (iv), we get
(40 + 20) x (6/60) = (40T)/ (60)
T = (360/40) = 9min