Question

Two trains 'a' and 'b' are moving on parallel tracks in opposite directions. The velocity of 'a' is twice that of 'b'. They take 18 seconds to cross each other. What are the velocities of trains a and b ? Take length of train a as 240 m and that of b as 300 m.

Answer 1

length of train a , l = 240m

length of train b , L = 300m

time taken to cross each other , t = 18s

Let velocity of train a is v1 and velocity of train b is v2

a/c to question,

the velocity of train a is twice that of b.

so, v1 = 2v2.

trains are moving in opposite directions.

so, relative velocity = (v1 + v2) = (2v2 + v2) = 3v2

so, time taken to cross each other is given by, t = (length of train a + length of train b)/(relative velocity)

or, 18 = (240 + 300)/3v2

or, 18 = 540/3v2

or, v2 = 10 m/s

and v1 = 2v2 = 20m/s

so, velocity of train a = 20m/s

so, velocity of train a = 20m/s and velocity of train b = 10m/s