Question

1 point
A 180 metre long train is
moving due north at a speed of
25 m/s. A small bird is flying due
south, a little above the train,
with a speed of 5 m/s. The time
taken by the bird to cross the
train is
O (a) 10 s
O
(b) 12 s
O (c) 9 s
O (d) 6 s​

Answer 1

Answer:

• Time taken by bird to cross the train = 6 seconds

Explanation:

Given that:-

• A small bird is moving due south just above a train.

• A train of length, s = 180 m is moving due north.

• velocity at which small bird is moving, v = 5 m/s

• velocity at which train is moving, u = -25 m/s

[Taking velocity of train negative because bird and train are moving in opposite directions]

We need to find:-

• Time taken by the bird to cross the train, t =?

Solution:-

[ In general, for two bodies A and B moving with velocities v and u respectively.

• The relative velocity of A w.r.t B = v - u  ]

Here, Let the relative velocity of Bird w.r.t Train be V, then

→ V = v - u

→ V = 5 - (-25) = 5 + 25 = 30 m/s

Now, as given that distance bird need to cover 180 m so Using formula

→ Time = distance / speed

→ t = s / V

→ t = 180 / 30

→ t = 6 sec

Therefore,

• Time taken by bird to cross the train is 6 seconds.

Answer 2

Given,

• Lenth of the train = 180 m, s(distance) = 180 m
• Speed of the train = 25 m/s
• Speed of the bird = -5m/s (as bird is in the opposite direction of train)

To find,

• Time taken by the bird to cross the train.

Formula used,

• Time = Distance/Speed

Solution,

According to the question, we need to find out the time taken by the bird to cross the train, so we need to know the sum of the speed of both bodies.

• Relative Speed = Speed of train - Speed of bird

${\implies}$ = 25 - (-5)

${\implies}$ = 30

Now, let's find out the time taken by applying the formula

• Time taken = Distance/Speed

${\implies}$ = 180/30

${\implies}$ = 6 seconds

Therefore, the time taken by the bird to cross the train is 6 seconds.