# Homework Help: Wave questions

1. Mar 15, 2005

### primarygun

I list the question one by one. I hope this is a good arrangment.
1.The blades of a fan is rotating in an anti-clockwise direction. Its motion is frozen with a strobe frquency of 1000 Hz without a change in appearance. Which of the following is not a possible rate at which the wheel is rotating?
There are six slits on the fan.
A.200 revolution per second.
B.500 rev. per second.
C.2000 revolution per second.
2.Why the wavelength of a wave increase when it passes from a shallow region to a deep region? Does the amplitude change?
3.Why the wavelength of a wave increase when it passes from a shallow region to a deep region? Does the amplitude change?
4. Why does the wavelength of a wave play a very important role in diffraction?
If the wavelength is bigger than the hole passing through, a very circular wave is produced while a very straight wave is produced if the hole is big enough?

2. Mar 15, 2005

### HallsofIvy

Have you read the 'sticky'? Show us what you have tried so we will know what kinds of hints you need.

3. Mar 15, 2005

### primarygun

OK. For the first one, actually, I suspect whether the choices are wrong.
For the other, the contents are not in my school's syllabus, so I don't have any idea.
Sorry!

4. Mar 16, 2005

### primarygun

Can anyone help me?

5. Mar 16, 2005

### Shockwave

6. Mar 16, 2005

### Andrew Mason

This means that in 1/1000th of a second, the fan as moved an even multiple of 1/6th turns. So turns/sec = 1000* n/6.

Or, more formally:
$\omega\Delta t = \omega/1000 = \theta = n*2\pi/6$
$\omega = 2\pi f = n2\pi*1000/6$.

So $f = n*1000/6$ where n is a whole number.

AM

7. Mar 17, 2005

### primarygun

May I know what w and pi and theta are?
I don't know where I archieve to the wrong way, please tell me.
1/1000 s for one revolution, actually, our view is only blocked for 1/6000 s.
So there are 1/6000 s for the n complete wave to pass.
So I think (1/6000)/T =n T, the time required for one complete wave, Period.
Then I get f=6000n. (n: integer)
Why?

8. Mar 17, 2005

### primarygun

OK. I get it. However, would you explain it to me again?
According to the rule here, I shall tell you first in order to obey the law. :P
In (1/1000)s, the fan moves, ---> the slits moves a multiple of 1/6 n round(a complete revolution like a complete wave). That's mean there are (1/1000)s for it to move.
Period= (1/6 n)/(1/1000)s

Last edited: Mar 17, 2005