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conantee
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เราเป็นอย่างไร สังคมเป็นอย่างนั้น

 « on: December 31, 2009, 06:17:45 AM »

12. A source of 1-kilohertz sound is moving straight toward you at a speed 0.9 times the speed of sound. The frequency you receive is
(A) 0.1 kHz
(B) 0.5 kHz
(C) 1.1 kHz
(D) 1.9 kHz
(E) 10 kHz
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conantee
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 « Reply #1 on: December 31, 2009, 06:27:02 AM »

This is doppler effect. When the source moves toward the receiver, the received frequency becomes higher than the frequency measured by the source. So, (A) and (B) can be eliminated.

Now, we need an equation to describe the relation between frequencies and speeds of source/receiver. The easiest way is to remember the formula, unfortunately:

, where the subscription s refers to source, and r refers to receiver. Assume the orientation that source is on the left, receiver is on the right. The velocity in equation is treated as vector (for example, moving to the right is positive value).

Plug all values we have. Then, we got
So, the frequency for receiver is 10 kHz.

The correct answer is (E). 39 of 100 people got the right answer.
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