Question

When a train approaches a stationary observer, the apparent frequency of the whistle is and when the same train recedes away from the observer, the apparent frequency is then the apparent frequency when the observer moves with the train is?

Answer 1

The apparent frequency when the observer moves with the train is 2n' + n''/n' + n''

Given-

• When train approaches a stationary observer the apparent frequency of the whistle is = n'
• When the same train recedes away from the observer, the apparent frequency is = n''

Now,

n' = vn/v-vs

n/n' = v-vs/v

n/n' = 1- vs/v -------(i)

And,

n'' = vn/v+vs

n/n'' = 1 + vs/v -------(ii)

On adding equation (i) and (ii) we get

n/n' + n/n'' = 2

So n = 2n' + n''/n' + n''