Question

How long will a boy sitting near the window of a train travelling at 36 km h see a train passing by in the opposite direction with a speed of 18 km h^-1.The length of the slow- moving train is 90 m.

Solution
The relative velocity of the slow-moving train with respect to the boy is = (36 +18)km h-1 = 54 km h^-1
= 54×5/18 ms^-1
=15 ms^-1

pls tell me how 5/18 came there ​

Answer 1

5/18 is used to convert km/hr into m/s

1 km = 1000 m
1 hr = 3600 sec

1 km/hr to m/s
1*1000/1*3600
1000/3600metre/sec
10/36metre/sec
5/18 metre/sec

Answer 2

Concept:

• Relative velocity
• One-dimensional motion
• Kinematics equations

Given:

• Velocity of boy in train = 36 km/h = 36 *1000/3600 = 36*5/18 = 10 m/s
• Velocity of train coming from the opposite direction = 18 km/h = 18 *1000/3600 = 18*5/18 = 5 m/s
• Length of the slow moving train

Find:

• The time for which the boy will see the slow-moving train pass by

Solution:

We have to use the concept of relative velocity and relative displacement.

Since both the trains are moving in opposite directions, their relative velocity is the sum of their individual velocities.

Vrel = 10 + 5 = 15 m/s

The relative displacement is equal to the length of the slow-moving train.

Relative displacement = 90 m

Time = displacement/velocity = displacement per unit time

T = relative displacement / relative velocity

T = 90/15 = 6 s

The boy will see the slow-moving train pass by for 6 seconds.

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