Homework Help: physics

Posted by PLEASE HELP!! on Thursday, February 3, 2011 at 8:58pm.

While blowing its horn of frequency 477 Hz, a car accelerates at 0.94 m/s2.

The car starts from rest by your side and moves away.

How many seconds does it take for the frequency you hear to decrease by 5%?

Hint: Do not forget to include the time it will take for sound to reach you.

*Im getting an answer of about 20, but i dont want to submit my homework yet, can someone please check it over to see if i am correct?*

physics - Henry, Sunday, February 6, 2011 at 12:23pm
F = ((V + Vr) / (V + Vs))Fo = 0.95*477.

((343 + 0) / (343 + Vs))477 = 453.15,
(343 / (343 + Vs))477 = 453.15 Hz,
163611 / (343 + Vs) = 453.15,
Cross multiply:
155430.45 + 453.15Vs = 163611
453.15Vs=163611 - 155430.45 = 8180.55,
Vs = 8180.55 / 453.15 = 18.05m/s.

Vf = Vi + at = 18.05m/s.
0 + 0.94t = 18.05,
t = 18.05 / 0.94 = 19.2s to reach final velocity.

d = Vi*t + 0.5at^2,
d = 0*19.2 + 0.5 * 0.94 * (19.2)^2,
d = 0 + 173.26m = 173.2m after final velocity.

d = V*t,
t = d / V = 173.2m / 343m/s = 0.50s to
reach receiver(you).

t(tot.) = 19.2 + 0.5 = 19.7s

So our answers are about the same.

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