Spring 2005

MWF 11:30

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

# REVIEW QUESTIONS OF Home Work 13

# R19.8. Period = time it takes for a wave to
complete one cycle. Amplitude = magnitude of the wave. Wavelength = length of a
wave before wave starts repeating itself. Frequency = number of cycles of a
wave that are completed per second.

# R19.10. Frequency = 1/period.

# R19.13. Speed = frequency x wavelength

# R19.14. For transverse waves the vibrations
are perpendicular to the direction of travel of the wave.

# R19.15. For longitudinal waves the
vibrations are parallel to the direction of travel of the wave.

# R19.18. For constructive interference (of
two waves of the same amplitude), the peaks of two waves line up and the waves
added to give a resultant wave which has twice as large an amplitude. For
destructive interference (of two waves of the same amplitude), the peak of one
wave lines up with the trough of the second wave – the two waves then
cancel each other out.

# R19.21. A node is a point on a standing wave
where one motion occurs. An anti-node is a point on a standing wave where
maximum motion occurs.

# R19.22. Frequency and wavelength both change
in the Doppler effect, however, the wave speed remains unchanged.

#

# EXCERISES OF Home Work 13

## E19.10. (a) transverse wave, (b) longitudinal wave, (c)
transverse wave.

## E19.11. She hears the sound of the gas long before
she smells the gas because the sound travels at a higher speed than the speed
at which a gas molecule diffuses to her nose.

**E19.12. If the
frequency is doubled (eg. from 1Hz to 2Hz) then the
period is halved (i.e. from 1s to 0.5s) because period = 1/frequency. **

**E19.13.
Frequency = speed/wavelength therefore the shorter wavelength light (i.e.
violet light) has the greater frequency.**

**E19.18. A wave
travels one wavelength in one period. **

**E19.25. There is a Doppler effect when the source of sound is
stationary but the listener is in motion because the listener would come across
the wave peaks sooner if moving towards the source (or later if moving away
from the source), hence the wavelength of the sound will change, compared with
if the listener was stationary. The wavelength would be smaller when moving
towards the source, therefore, the frequency (= speed/wavelength) would be
higher. The wavelength would be larger when moving away from the source,
therefore, the frequency (=speed/wavelength) would be lower in this case.**

**E19.26. (a)
Wavelength is smaller because the train is moving towards you, therefore, the
frequency (= velocity/wavelength) is larger i.e. the frequency increases, (b)
the wavelength decreases, (c) the speed of sound stays the same. **

**E19.30. It means that the sun is spinning on its axis. As the sun
spins, one edge is moving towards you and the wavelength of light is slightly
smaller due the Doppler effect; hence, for this edge you detect a higher
frequency (= velocity/wavelength). For the other edge of the sun (which is
moving away from you), the wavelength of light is slightly larger; hence, for
this edge you detect a smaller frequency (= velocity/wavelength). **

** **

# PROBLEMS OF Home Work 13

# P19.3. Wave crests pass anchor every 5s,
therefore, the period (i.e. time for one cycle) = 5s. Frequency = 1/period =
1/5s = 1/5 Hz. Wavelength = 15m.

# Speed = frequency x wavelength = 1/5 x 15 =
3m/s.

**P19.4.
Frequency = 2/s = 2 Hz. Period = 1/frequency = 0.5 s. **

Amplitude = 10 cm.

**P19.6.
Frequency = 600 Hz. As the speed of sound in air = 340 m/s, therefore, **

wavelength = speed/frequency = 340/600 = 0.57 m.

**P19.7.
Frequency = 256 Hz. (a) Period = 1/frequency = 1/256 = 0.0039s. (b) Speed =
340m/s. Wavelength = speed/frequency = 340/256 = 1.33m.**

** **