Question

Because of the Doppler Effect, an observer behind a moving wave source observes A. increased wave frequency B. no wave frequency C. decreased wave frequency D. unchanging wave frequency

Answer 1

if source is moving towards the observer, then increased frequency. else, decreased.

Answer 2

thank you!!

Answer 3

according to doppler effect:
1)when the distance between source and observer decreases then f'=f*((V+Vo)/(V-Vs))
where f' is apparent frequency and f is the frequency of the source ,
Vo->velocity of observer.
Vs->velocity of the source
V->velocity of the sound
ii)when the distance between source and observer increases then f'=f*((V-Vo)/(V+Vs))
you must keep these concepts in the time of solving the problems which is based on doppler Effect ......match your answer..apply above concepts.
If source is in rest put Vs=0;
if observer is in rest put Vo=0 in the above formula ....
Note if both are in rest then there is no change in frequency... if they are not in rest then frequency may increase or decrease...which has been discussed above in 1) and ii) case ..