Spring 2005
MWF 11:30
Instructor: Dr. Bruce Law, CW
327, Tel: 532-1618.
REVIEW QUESTIONS OF Home Work 14
R20.3. Frequency and pitch are identical for
sound waves.
R20.6. Compression = region where the
particles are closer together in a sound wave. Rarefaction = region where the
particles are further apart in a sound wave.
R20.10. Sound will not travel through a
vacuum because it needs a medium to travel in.
R20.13. Sound travels faster in warm air
compared with cold air because the air molecules are traveling faster in warm
air.
R20.25. In order to make an object resonate,
one needs to excite the object at one of it`s
resonance frequencies. (Remember an object has many resonance frequencies.)
R20.28. One wave will cancel another wave if
the peaks of one wave line up with the troughs of the other wave and where both
waves have the same amplitude.
R20.30. Beats occurs when two waves have almost
identical frequencies. When two peaks (of the two waves) line up then one gets
constructive interference, however, if a peak of one wave lines up with a
trough of another wave then one gets destructive interference. The result of
this is that one will hear regions of constructive interference followed by
regions of destructive interference and the amplitude of the resultant wave
will increase and then decrease with time – this is the phenomenon of
beats.
EXCERISES OF Home Work 14
E20.3. Wavelength = speed/frequency, the
larger the frequency the shorter the wavelength. Bats therefore hear shorter
wavelengths than cats.
E20.5. Wavelength = speed/frequency. Frequency
of source A, fA = 2 x fB.
Therefore
wavelengthA = speed/fA = speed/(2 x fB) = wavelengthB/2 where wavelengthB
= speed/fB. Hence the wavelength of source A is half the wavelength of source B.
E20.6. Wavelength = speed/frequency. The frequency is the same. The
electromagnetic wave (light wave) has a higher speed and therefore longer
wavelength than a sound wave.
E20.12. Light
can travel through a vacuum. Sound needs a medium to travel in.
E20.16. The speed is the same. The wavelength = speed/frequency is
halved.
E20.24. (1)
Light travels faster than sound therefore you should see the explosion before
you hear it and (2) sound cannot travel through a vacuum (i.e. space) therefore
in fact you should not hear the explosion!
PROBLEMS OF
Home Work 14
P20.1. Wavelength = speed/frequency = 340/340 = 1m.
Wavelength = speed/frequency = 340/34000 = 0.01m.
P20.3. Speed = 1530m/s. Time delay to the ocean floor
and back = 6s, therefore, it takes 3s for the sound to reach the ocean floor
and 3s for the sound to return. Speed = distance/time therefore distance =
speed x time = 1530 x 3 = 4590m is the depth to the ocean floor.