Water Wave

Eternal Snow

2010-03-16 8:06 pm

A continuous wave train moving to the right is frozen by a stroboscope. What is the change in speed of rotation of the stroboscope so that the wace seems to move slowly (1) to the right (2) to the left?

(1) (2)
Increased a little halved
Increased a little decreased a little
Decreased a little increased a little
Decreased a little doubled

I am not clear with the idea of this question. May you help me to explain the details? thanks!

更新1:

it should be (1) A.Increased a little B.Increased a little C.Decreased a little D.Decreased a little _________________________________________________________________________ (2) A.halved B.decreased a little C.increased a little D.doubled

回答 (1)

天同

2010-03-17 5:56 am

✔ 最佳答案

The answer is option C

Suppose the wave train that is travelling to the righ has a period of, say, 1 s (just for example). When the period of the stroboscope is also 1 s, you would see the wave frozen. It is because the position of the crest that you saw 1 s early is now replaced by the crest from behind.

Consider now if the period of the stroboscope is increased to 1.1 s (i.e. speed decreases slightly), then the crest from behind is no longer at the original position of the crest 1s earlier (the earlier crest), but has moved a little bit forward from that position. Hence, what you observe is the crest moves foward slowly.

If the period of rotation of the stroboscope is decreased slightly, say to 0.9 s (i.e. speed of rotation slightly increases), it is clear that the crest from behind has not yet reached the original position of the earlier crest. What you see is that the crest moves a lillte bit towards the left.

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