phys - sound

Ellen

2008-07-25 9:19 am

fig(a): http://good-times.webshots.com/photo/2355156350103642988skfdKj
fig(b): http://good-times.webshots.com/photo/2417540210103642988TRYcrv
A plane sound wave travels horizontally through the air. The propagation direction is along a line joining the east and the west. The air particles vibrate about equilibrium positions when the sound wave passes through them. Fig(a) shows the variation of the displacement iof the air particles with distance when time begins to be taken (i.e. time = o). Fig.(b) shows the variation of the displacement of the air particle at position b with time.
1. Which of the following position(s) has/have a compression at time equal to zero?
A. a and e B. b C. c D. d
2. Which of the following position(s) has/have a rarefaction at time equal to zero?
A. a and e B. b C. c D. d

p.s. Please describe detailedly.

更新1:

How can I know which direction of the sound wave travels towards?

更新2:

Would you tell me more, what the movement of particle a and particle c are?

回答 (1)

天同

2008-07-26 4:23 am

✔ 最佳答案

1. B
Particles to the left and right sides of point b are moving towards it, thus forming a compression at b

2. d
Particles to the left and right sides of point d are moving away from it, thus forming a rarefaction at point d.

Q:How can I know which direction of the sound wave travels towards?

The wave is travelling from east to west. Just look at the subsequent motion of point b from fig(b), which is downward. This shows the wave trough is approaching point b. Thus the wave, as seen from fig(a), is moving towards the left.

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