# All you physics buffs out there

Discussion in 'Off Topic [BG]' started by Gunnar Þór, Apr 11, 2003.

1. ### Gunnar ÞórGuest

Sep 23, 2001
Hafnarfjordur, Iceland
Little question for you guys,

Code:
```A           B
|\        / |
|  \    /   |
|    \/     |
|    / \    |
|   /    \  |
| /        \|
S1        S2```

S1 and S2 are the points of origin of two waves in the same phase, A and B are the points where the waves cancel out. Between points A and B are possible additional points* where the waves also cancel out.

Is there a formula for calculating the wavelength of the waves, if all distances are known?

*Called "knots" in Icelandic if it tells you anything

2. ### Jeff MooteSupporting Member

Oct 11, 2001
To find the wavelength, you want to take two times either S1 to A, or S2 to B. Those distances are both half a wavelength.

Oh wait! You say same phase? I don't quite understand, but maybe your diagram confuses me.

Hmm... what you say are "knots" are probably what we call nodes. In a standing wave (in a medium, i.e. string on an instrument), nodes are the places where there is no vibration.

So are points A and B not the same place, or close enough to be negligible? I just don't quite understand the question.

3. ### Milothicus

The places they cancel out are called nodes in english. or nodal points.

none of my current textbooks seem useful in this case, but if you search for nodes or nodal points you might be able to find something online.

4. ### Jeff MooteSupporting Member

Oct 11, 2001
Another way to do it, is to just deal with the math, if you knew the speed of the wave, and it's frequency. The universal wave equation is:
Code:
`--blank--`
or velocity (speed) equals frequency times wavelength. So rearranged to isolate wavelength you have:
Code:
`--blank--`
Maybe that will help.

edit: characters don't work... making image to help.

5. ### Jeff MooteSupporting Member

Oct 11, 2001
hopefully this helps...

these were supposed to be in the code in the last message:

6. ### Milothicus

i don't think there is a way to know exactly, because having a node after 10 vertical waves will look exactly like having a node after 20 waves of half the original wavelength, and you'll never know the difference unless you know how many waves there are. but.............

if x is the horizontal distance, and y is the vertical distance, and w is the wavelength (no lambda on my keyboard)

w*i= (x^2 + y^2)^0.5 - y

where i is a postive integer.

explanation:

you know there are N waves between S1 and A, and M waves between S2 and A. both N and M are whole numbers, because it's a nodal point.

so...........since the sources are in phase, the difference between the two distances travelled must be (M-N) waves. therefore, the difference between the distances has to be some positive whole number multiple of the wave length.

2nd edit:

hm....still something wrong. you could just use either original distance as a whole-number multiple of wavelength..........

7. ### soundoholic

Jan 22, 2003
Rapid City, SD
I never studied in physics. I was too busy playing in bands. But I can tell you that it was a cool class!

8. ### Milothicus

of course, everything i said in my previous post assumes a node when the waves meet in phase, which is wrong. for a node, you need destructive interference, which means one at a maximum and one at a minimum...........

in short, i still don't know......

9. ### BigBohn

Sep 29, 2001
WPB, Florida

We must have been readin out of the same textbook cause that's what I got too.

10. ### Gunnar ÞórGuest

Sep 23, 2001
Hafnarfjordur, Iceland
MontyP: Yes, I mean nodes, thanks. Points A and B can be anywhere but let's assume they're not in the same place. They are however at points where the waves cancel each other out, nodes. Dealing with the math isn't a problem but in this case the wavelength, frequency and speed are all unknown. By in the same phase I mean they're transmitting at the same wavelength and at the same time.

Milothicus: True, the distance from AS2 minus AS1 is a multiple of a n+1/2 times lambda. Same goes for B. But lambda is what we're trying to find anyway. The waves meet at points A and B and destroy each other, and at a point between A and B.

Josh: Actually where I got this from the answer was 8.

I'll post the original problem:

Two speakers, S1 and S2, are seperated by 40 centimetres. They transmit waves in the same phase with an unknown frequency. At point A which is 150 cm from S1 and 162 cm from S2 the waves cancel out. At another point, B, which is 140 cm from S1 and 168 cm from S2 the waves also cancel out. Between points A and B is another node where the waves cancel out. What is the wavelength of the sound?

This was then solved like this: 162 - 150 = 12. 168 - 140 = 28. From those numbers you were then supposed to see that 12/1.5 = 8 and 28/3.5 = 8. Therefore, the wavelength is eight cm.

11. ### Jeff MooteSupporting Member

Oct 11, 2001
That's interesting... I think some of the confusion came from an assumption I (and possibly others) made from your original info that they were travelling parallel.

I dunno... I tried. Bleh.

12. ### Freaky Fender

May 27, 2001
Saunderstown, RI
I got 7.29... maybe i counted wrong..

13. ### Gunnar ÞórGuest

Sep 23, 2001
Hafnarfjordur, Iceland
All right, thanks guys. I was asking because a couple of us found a way of calculating it with no limit on the number of nodes and without knowning the frequency, the only thing you need to know is the distances from the speakers to the points and how many nodes there are between them. We're just trying to find out if it's been done before.

14. ### neptoonSupporting Member

Jul 25, 2000
summerville, sc

you're all wrong....the answer is 42

15. ### Milothicus

and the question is "What is six times nine?"

16. ### CAM

Mar 10, 2003
mongo's house
jesus h christ!!! you guys are talking in another language man!!! physics is cool so rock on!! but ill be damned if i understand a word of that!! Speed = distance x time!! hehe

17. ### Gunnar ÞórGuest

Sep 23, 2001
Hafnarfjordur, Iceland
All matter is energy. Chicks dig it.

18. ### Jeff MooteSupporting Member

Oct 11, 2001
laf, actually speed = distance / time

meh, physics is cool.

19. ### SheepHerder7898

Jun 15, 2002
Chesapeake, VA

20. ### Zoroaster03

or, "If you had nine 6 string basses, how many strings would you have in total?"