Phys 2110, Fall 2008
Problem Set #14
14.17 Use the basic equations relating λ,fand v.
14.20 Use the distance and time to get the wave speed. (Yes, hee speed is distance divided
by time.) The get the wavelength (this is a harmonic wave).
14.23 In our usual formula for the harmonic wave, the coefficient of xis kand the coefficient
of tis ω.
14.27 Here the “spring” works the same as a streched “string”. Find the string tension from
v=qF/µ. Watch the units.
14.29 Get the wave speed from v=qF/µ. Use the formula for power ¯
Ptransmitted by a
wave on a string.
14.57 Since the power is the same in both cases, you can use
P=I(4πr2) = const =⇒I1r2
1=I2r2
2
to find the second distance more easily. Note how the problem is worded: How much further
do you need to walk away.
14.66 Here the fundamental frequency of the string plays, so the wavelength is twice the
length of the string. Find the speed of waves on the string. Get the mass density and then
the mass of this particular piece of string.
Watch the units.
14.65 Here the sound source is an isotropic source, with I=P /(4πr2). You can use the first
numbers to get the intensity of the wave at the first distance and from that get P, then from
the second intensity (also gotten from β) find the second distance. Or use Ir2=constant for
a point source.
This time the question is asking for the total distance from the source at the second
point.
14.38 The longest wavelength standing wave on the string has a wavelength twice the length
of the string.
14.43 Watch the units. Here the observer is in motion toward the source. He should then
hear a higher frequency. As far as I know, you should use 343 m
sfor the speed of sound.
14.69 Here you don’t know the original frequency of the sound; you know that the car’s
speed stays the same but initially the motion of the source is toward you and after it’s away
from you.
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