# Help with physics wave problem

Shay10825
Hello everyone! I need help with some problems my physics teacher gave us. The problems I did not show any work for are the ones I have no clue how to do. I would just like a hint or something to get me started (I don't want you to do the whole problem for my because I need to know how to do it myself). I would like to know what I did wrong for the ones I showed work for but I don't want you to do the whole thing. The work is either typed or written. The correct answers are circled or typed. I'm sorry this is so long.

5.) http://img117.exs.cx/img117/6494/number51oa.jpg

7.) http://img117.exs.cx/img117/6541/number72mb.jpg

11.) http://img117.exs.cx/img117/8550/number110wo.jpg

13.) http://img120.exs.cx/img120/6853/number132rd.jpg

14.) http://img120.exs.cx/img120/3610/number14pt12qr.jpg
http://img120.exs.cx/img120/9120/number14pt25rq.jpg
The answer is: A. pi

15.) http://img120.exs.cx/img120/2436/number153bq.jpg

16.) http://img120.exs.cx/img120/3938/number168gf.jpg
The answer is: C. 120

work:
f= 1/(2Lv)
60=1/(6v)
v=.0027

19-22.) I don't know why I scanned 19. I don't need help with 19.
http://img53.exs.cx/img53/1495/number19222qi.jpg

20 work:
f= 1/(2Lv)
so why is the answer .25?

The answer for number 21 is: B. pi

23.)http://img120.exs.cx/img120/6513/number230el.jpg
http://img149.exs.cx/img149/416/number23pt24ij.jpg

The answer is: D. L/8

24 - 27.) http://img149.exs.cx/img149/1576/number24279ia.jpg

For number 24 why is the answer not f= 1/(2Lv)?

30 - 32.) http://img149.exs.cx/img149/2849/number30329am.jpg

How did they get the -10 I number 30? Would you integrate to get the answer? If so how did they get the -10?

For number 31 would I use the formula v=sqrt(T/u)? If so how do I get the values of T and u??

In number 32, I got (400,0) as a point on the graph. So it this why the answer is 4?

34.)http://img149.exs.cx/img149/1332/number343hc.jpg
Is the book wrong for this answer?

Any help would be greatly appreciated.
~Thanks

Last edited:

## Answers and Replies

Staff Emeritus
Gold Member
Boy, does this look scary.

5) I don't get why you write f=168 and then pick one of the choices that does not include this number.

The condition for resonance is that the far (closed) end of the pipe have a node of the wave.

So, for any integer n, you have resonance when $$L = \frac {n \lambda}{2}$$

Now use the relation for the speed of sound, and after plugging in numbers you should get something that looks like f = nF, where F is a number you get from the calculations. This implies that all frequencies that are multiples of this number F, will result in resonance. Now from the choices, find the set that has only multiples of this F.

Staff Emeritus
Gold Member
7) I don't get how you did this. You've written f = 520(2/340). This is not correct. Also, you've written Vs = 340. That's not correct either. Check your text to see what each of the symbols means in the doppler formula.

The wall is acting like a mirror, bouncing back the sound to the flautist. A clever way to solve this problem is to actually treat the wall as a mirror, and treat the image (on the other side of the mirror) as the observer. So, if the man is moving at 2 m/s towards the mirror, what will be the speed of his image, and in which direction will it travel ? Now you can simply forget about the mirror. The man is the source, and his image is the observer. This is a direct substitution problem now.

Staff Emeritus
Gold Member
11) Still can't see how you're doing this. What's the formula for the speed of a wave in a stretched string?

Staff Emeritus
Gold Member
13) This is just one step more than the previous problem. Having found the speed, you can find the frequency if you know the wavelength of the first harmonic. What must the wavelength be ?

Shay10825
Gokul43201 said:
11) Still can't see how you're doing this. What's the formula for the speed of a wave in a stretched string?

When I first did the problem I got 200.

v= sqrt(T/u)
v= sqrt(600/.015)
v=200

but the answer is 632

Staff Emeritus
Gold Member
Shay10825 said:

When I first did the problem I got 200.

v= sqrt(T/u)
v= sqrt(600/.015)
v=200

but the answer is 632

Check the units on the linear density (mass per unit length) $\mu$

Shay10825
Gokul43201 said:
13) This is just one step more than the previous problem. Having found the speed, you can find the frequency if you know the wavelength of the first harmonic. What must the wavelength be ?

ok i know what i was doing wrong now. Thanks

Last edited:
Shay10825
Gokul43201 said:
Check the units on the linear density (mass per unit length) $\mu$

Wow I'm really stupid. Thanks

Shay10825
Gokul43201 said:
Boy, does this look scary.

5) I don't get why you write f=168 and then pick one of the choices that does not include this number.

I circled the correct answer. I see what I was doing wrong now. Thanks .

Shay10825
For number 16:

wavelength = (2/harmonic number)L
Would the harmonic number be 3? If so why.