The Physical World I

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.