# The Physical World I

Spring 2005
MWF 11:30

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

# EXCERISES OF Home Work 13

## 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).

# 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.