The Physical World I

Spring 2005
MWF 11:30

Instructor: Dr. Bruce Law, CW 327, Tel: 532-1618.


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.  



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.