Question

Two trains 120m and 80m in length are running in opposite directions with velocities 42km hr and 30km hr respectively in what time will they completely pass each other?

Answer 1

You need to use the concept of relative velocity here. The relative velocity in this case will be 42+30 as both the trains are running in opp direction. Also the distance needed to be traveled will be 120+80=200 m = 0.2 Km.

Time = Distance/Speed = 0.2/72 = 0.0027 Hr = 10 Seconds

Answer 2

Given:

• Train length is 120 metres and 80 metres
• Thy are travelling in opposite direction
• Velocities are 42 km/hr and 30 km/hr

To find:

• Time taken to completely cross each other?

Calculation:

• First of all, lets calculate the total length of the 2 trains and the relative velocity between them.

Total length = 120 + 80 = 200 metres

Relative velocity will be addition of the individual velocities because the trains are travelling in opposite direction.

v_(rel) = 42 + 30 = 72 km/hr

• We know 1 km/hr = 5/18 m/s

So, v_(rel) = 72 * (5/18) = 20 m/s

Now, time taken is :

$\implies time = \dfrac{total \:length}{v_{rel}}$

$\implies time = \dfrac{200}{20}$

$\implies time = 10 second$

So, time taken for the trains to cross is 10 seconds.