A man is hovering over a pit and a boy falls past with terminal velocity. The frequency of the boy's scream decreases from 842Hz to 820Hz. (a) Find the speed of descent of the boy.
(b) The boy's scream reflects from the bottom of the pit. Find the frequency of the echo as heard by the boy.
(c) Find the frequency of the boy as heard by the man?
Please I would like an explicit approach. Thanks.
2006-06-21 07:29:35 · 4 answers · asked by Stavi 2 in Science & Mathematics ➔ Physics
The timeframe starts from when the boy is at the same height with the man and continues as he falls to the bottom of the pit. The man observes that the frequency of the scream reduces from 842Hz to 820Hz during this timeframe.
2006-06-21 08:56:57 · update #1
a)
(change in frequency)/(recorded frequency) = (Speed of emitter)/(wave speed).
Change in frequency = 22 Hz
Recorded frequency = 820 Hz
Speed of emitter = what you are trying to find.
Wave speed = speed of sound = 340 m/s
(22 / 820) x 340 = 9.12 m/s
b)
The frequency of the echo is (I'm guessing for this one sorry) double the regular frequency, which is 1684 Hz.
c)
The frequency heard by the man is 820 Hz by definition isn't it? Thats the frequency as the boy passes anyway.
[EDIT] Wait I just realised your whole question makes no sense. Due to the limited info in the question I was assuming you meant it falls to 820 Hz from the emission frequency at the point where the boy passes the man. But why would the frequency fall? The point where the boy passes the man is the point where the frequency experienced by the man is highest.
Oh well I hope my answer helped a bit anyway.
2006-06-21 08:01:54 · answer #1 · answered by Anonymous · 1⤊ 0⤋
As Robert B says, you should try to work it out yourself. However, I suspect the 842 Hz includes the positive doppler shift (nu-zero plus delta-nu) and the 820 Hz includes the negative doppler shift (nu-zero plus delta-nu). Just as a train approaching has its frequency suddenly change from high frequency to low, the boy falling past the man would have the same instantaneous effect as his velocity toward the man suddenly changed to a velocity away from the man.
2006-06-21 20:26:59 · answer #2 · answered by volume_watcher 3 · 0⤊ 0⤋
a) isn't terminal velocity always the same, like 33 ft/sec^2 ore something like that? I might be wrong?
b) the kid is going to die. He doesn't care about the frequency
c)842-820 Hz, as you said. Depends on his position.
2006-06-21 14:33:57 · answer #3 · answered by Schmorgen 6 · 0⤊ 0⤋
http://en.wikipedia.org/wiki/Doppler_effect
I did this over thirty years ago as a Radar Engineer which at that time used Doppler
Now you can work it out like I had to
2006-06-21 14:36:09 · answer #4 · answered by Robert B 4 · 0⤊ 0⤋