Question

Q21. Mary is riding a roller coaster. Her mother who is standing on the ground behind her yells out to her at a frequency of 1000 Hz, but it sounds like 920 Hz. (Vsnd 343 m/s). Which one is correct about the problem and Mary's speed? A) Moving observer away from the source with the speed 27.4 m/s B) Moving source away from the observer with the speed 2.74 m/s c) Moving source towards the observer with the speed 27.4 m/s D) Moving observer towards the source with the speed 27.4 m/s E) Moving observer away from the source with the speed 2.74 m/s

Answer 1

Here. Mary is observer, mother is a source of sound Actual frequency of sound = nu_0 = 1000 Hz Apparent Frequency of sound = nu' = 920 Hz velocity of sound = V_sound = 343 m/s Let Mary's speed = V_0 = ? it is leased on doppler effect, which state that whenever there is a relative motion between observer and source of sound, Frequency of sound as observer will be different from the actual freq. Now since final frequency is less than the actual therefore separation between source and observer should be increasing. there are two cases, either source is moving away on the observer is moving away. In this case her mother is still, but mary is moving away on roller coaster. Using doppler's formula therefore apparent frequency = (V_sound - V_0 /V_sound) times actual frequency \r\n f' = (V_s - V_0 /V_s) f f'/f = V_sound - V_0 /V_sound 920/1000 = 343 - V_0 /343 920/1000 times 343 = 343 -