# Homework Help: Interference of waves (check my hmwk questions please)

1. Dec 8, 2011

### supernova1203

For the first question, number 8 i have put up 2 attachment, one with the question the other one with the answer, im not sure of the answers however, so if someone can check to see if theyre right or not, and if not show me how to attain the right answer.

thanks

For the next question, number 11, ill just show you the question and do the solution right here.

11)a) f1= 320 Hz
f2=?

f=# of cycles/total time

f=14/7

f= 2Hz (this 2 hz is also the beat frequency)

fb is beat frequency

fb= f2-f1 theres a vertical line for absolute value but i dont know how to type it in

f2= f1-fb

f2=320 + or - 2

f2= 322 hz or f= 318 hz

that was 11a

now 11b

11b) f1=320 hz
fb=1 hz(the beat frequency if you looked at question is now 1 instead of previous 2 hz)

fb = f2-f1

f2=f1-fb

f2= 320 + or - 1

f2= 319 hz or f2=321 hz

Now how do i know which is the original frequency of tuning fork 2? i dont know what to do next, i dont know which is the right frequency for tuning fork 2.

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2. Dec 8, 2011

### supernova1203

Re: Interference of waves (check my hmwk questions pls)

anyone?

3. Dec 8, 2011

### PeterO

Re: Interference of waves (check my hmwk questions pls)

Adding the rubber bands increases the mass of the fork which slows it down, so the second frequency is smaller than the first.

4. Dec 8, 2011

### PeterO

Re: Interference of waves (check my hmwk questions pls)

I didn't like the look of you superposition.

It appears that one pule is trying to take the string down, while the other is trying to take the string up. Net result can tend to be very little.

You are adding these pulses [looks like you may have subtracted].

Given that the "down" pulse is a square pulse, you can simply translate the "up" pulse lower.

I suspect a slight up result at the left, and then a mainly down result at the RHS.

5. Dec 14, 2011

### supernova1203

Re: Interference of waves (check my hmwk questions pls)

hm.. ok i tried the super positioning question again, let me know what you think, and if i dont have it right can you draw it on paint and post it here so i can see what you did, and preferably how you did it?

(new attempt in new attachment)

this is the new attempt at question 8, the very first attachment from original question.

an explanation is in order i suppose, the superposition looks the way it does because negative and positive values are assigned to top and bottom, since there is more at bottom half, we assume there is a higher negative value attached to it, and so i basically shifted it into the negative area(which is below the line or rest position)

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• ###### Best attempt super positioning.jpg
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6. Dec 14, 2011

### PeterO

Re: Interference of waves (check my hmwk questions pls)

I agree with the principle of your answer, but when I look at the original problem, I think the upward/sloping pulse has a maximum which is larger than the downward/square pulse, so I think the sloping resultant you are drawing should start out a little upward and slope down the way you show.

If I were to put values on the pulses - for explanation purposes - I would say the up ward pulse has a peak value of 3, sloping down to 0, while the downward pulse is a steady -2. The resultant would start at a peak value of 1, sloping down to -2.

Perhaps those values are a little extreme [in comparison] and It should have been 5 → 0 for the upper pulse, and -4 for the lower, with the resultant going from 1 → -4.

Those values are arbitrary, just used to show relative size of the pulses.

7. Dec 15, 2011

### supernova1203

Re: Interference of waves (check my hmwk questions pls)

the problem is that in the course so far they havent assigned any numerical or mathematical values to any of these waves or their superpositions, they havent even mentioned them, they just expect me to somehow solve them visually just be looking at them, and without numbers or mathematics thats the best i can come up with, however if they did use mathematics/numbers i would have been able to solve them much more accurately and easily.

8. Dec 15, 2011

### PeterO

Re: Interference of waves (check my hmwk questions pls)

That is OK, but I used the numbers - because I can't draw on this forum - as a way of describing what I thought the diagram looked like. The higher end of the upward pulse looked bigger than how far the lower pulse goes down, so the resultant should have a small upward part to the pulse at the left end.

If you have the diagram on paper, you could actually use a ruler and measure the size. If only electronic, you could copy it into "paint" or similar and drag the image around and compare it for size.

9. Dec 15, 2011

### PeterO

Re: Interference of waves (check my hmwk questions pls)

I did the "paint" thing and copied the left hand end of the lower pulse into the middle of the upper pulse, confirming that the peak of the upper pulse is bigger than the lower pulse.
If I understand how this works, the attachment should be here as well.

EDIT: That worked well. You could actually do the supoerposition in paint, by copying the upper pulse and pasting it onto the lower pulse in a similar way.

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