Question

Answer for cbse class 11 1. Two trains each of length 256 m are moving in opposite directions on two parallel railway lines. The speed of one is twice that of the other. They take 4 sec to cross each other. Find their velocities

Answer 1

Answer:

let the speeds of the trains be A and B and

their length be L

A = L/4

B = L/5

now when they move in same direction

to cross one train, the other train has to cover 2L distance

now

relative velocity = L/4 - L/5 = L/20

therefore time taken = 2L/L/20 = 40 sec

Answer 2

Two trains of length 100m and 250m run on parallel lines. when they run in the same direction it will take 70 sec to cross each other and when they run in opposite direction, they take 10 sec to cross each other. Find the speed of faster train.

A. 10 m/s

B. 15 m/s

C. 20 m/s

D. 25 m/s

E. 30 m/s

Spoiler: OA

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Most Helpful Expert Reply

VeritasKarishma

EXPERT'S

POST

Mar 29, 2012

manojgmat wrote:Two trains of length 100m and 250m run on parallel lines. when they run in the same direction it will take 70 sec to cross each other and when they run in opposite direction, they take 10 sec to cross each other. find the speed of faster train.

Alternative approach:

To cross each other (either in same or opposite direction), the trains have to cover a distance of 250 + 100 = 350 m (the faster train should cover the entire slower train and then its own length so that they completely cross each other)

When they run in the same direction, they cover this distance in 70 sec. So their relative speed in this case (which is the difference in their speeds) is 350/70 = 5 m/s

When they run in opposite directions, they cover this distance in 10 sec. So their relative speed in this case (which is the sum of their speeds) is 350/10 = 35 m/s

If sum if 35 and difference is 5, you should quickly jump to 20 and 15.

Note: We used the concept of relative speed here.

When 2 objects move in same direction, their relative speed i.e. speed relative to each other is the difference of their speeds.

When the 2 objects move in opposite directions, their relative speed i.e. speed relative to each other is the sum of their speeds.