# Physics Questions!

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An interference pattern is set up by two point sources of the same frequency, vibrating in phase with one another. A point on the second nodal line is 50.0 cm from one source and 59 cm from the other. The speed of the waves is 35 cm/s. Calculate the frequency of the sources. I know you do path difference (like Pms-Pms) and then do the universal wave equation, I am confused where it says in phase with one another and the 2nd nodal line

• Physics Questions! -

The "first nodal line" is a line along which the path difference is 1/2 wave, a point of destructive interference. In this case, 9 cm is the wavelength, since there would be one extra wavelength in path difference along the second nodal line.

The frequency is then f = (35 cm/s)/9 cm = 3.9 Hz. Those are very slow waves, by the way. Are you sure you got the units right?

It does not matter whether the two sources are in phase with one another or not. It affects where the nodes are, but not the separation between the nodes.

http://www.glenbrook.k12.il.us/gbssci/phys/Class/light/u12l3a.html

• Physics Questions! -

mmmh are you sure it is 9 cm? It is destructive interference which means a minimum, therefore PmS1-PmS2= (m+1/2) lambda...I get 9 cm when I don't account for the m + 1/2

• Physics Questions! -

You are correct. 9 cm is (3/2) wavelength along a second nodal line. The wavelength is therefore 6 cm.

• Physics Questions! -

Thanks for all of your help, I talked to my other friend who had originally posted, and we both appreciate this, thanks!

• Physics Questions! -

Sorry about the error. I'm glad you and BobPursley were on the ball. I was thinking of the path length difference between the first and second nodal line.

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