Question

Two trains move towards each other with the
same speed. The speed of sound is 340 m/s. If
the height of the tone of the whistle of one of
them heard on the other changes 9/8 times, then
the speed of each train should be
(a) 20 m/s (b) 2 m/s
(c) 200 m/s (d) 2000 m/s

Answer 1

Answer:

A)

Explanation:

According to Doppler effect the the pitch of a sound change due the motion of source is given by:

n

ˉ

=n

v−v

s

​

v−v

0

​

​

Here, v

s

​

and v

0

​

are the velocity of the sound source and observer and v=330m/s and

n

ˉ

/n=9/8. Lets assume the velocity of both the the trains are

v

ˉ

. Using the above formula:

n

ˉ

/n=9/8=

v−

v

ˉ

v−(−

v

ˉ

)

​

. The negative sign used for the opposite direction movement of the trains. Using all the known values we can get the velocity of the trains are 20m/s