In: Physics
What are the three longest wavelengths for standing waves on a 240 cm long string that is fixed at both ends?
If the frequency of the second-longest wavelength is 50.0 Hz,
what is the frequency of the third-longest wavelength?
a)
For a standing wave if both ends are fixed,
the wavelength must
be such that the length of the string must be an exact multiple of
half a wavelength.
The longest wavelength must be such that the
length of the string must be equal to half a wavelength, and
therefore the wavelength must be double the length of the string,
or 2*240 cm = 480
cm.
The second longest wavelength must be such that
the length of the string must be equal to a whole wavelength, so
the second longest wavelength must be 240 cm.
The third longest wavelength must be such that the
length of the string must be equal to 1.5 times the wavelength, so
the wavelength must be 240cm/1.5 = 160 cm.
b)
The
speed
is the same for all standing waves. And remember v = f?, speed
is
frequency
times
wavelength.
The speed of the third longest wavelength is the same as the speed
of the second longest wavelength, which is
v = f? = (50.0 Hz)(2.4 m) = 120 m/s, and dividing by the wavelength
of the third-longest (1.6 m), gives a frequency of =75
Hz.