A source of sound S is travelling at `(100)/(3)(m)/(s)` along a road, towards a point A. When the source is 3 m away from A, a person standint at a point O on a road perpendicular to AS hears a sound of requency `v'`. The distance of O from A at that time is 4 m. If the original frequency is 640 Hz, then the value of `v'` is (velocity of sound is `340(m)/(s)`)

Options

A. 620 Hz
B. 680 Hz
C. 720 Hz
D. 840 Hz

Answer

B

Explanation

Effective value of velocity of source is
`v_S=(100)/(3)costheta`
`=(100)/(3)xx(3)/(5)=20(m)/(s)`
`v'=(v)/(v-v_S)v`
`v'=(340)/(340-20)xx640Hz=680Hz`