PHYS 140
Standing Waves on a String – Lab
You’ll need to work with a partner on this lab, so everyone should work in a group of 2 or 3 students.
Experiment 1: Vibrating string with elastic cord [white string].
Part 1
In this part, the string is vibrated at a constant frequency of 60 Hz. The speed of the wave is controlled
by the tension, since for a string v = sqrt(F/). (The force “F” is the tension, is mass per length.)
f = nv/(2L) for a string fixed at both ends.
1. Make sure that the power cord, not the sine wave generator, is connected to the input on the
string vibrator. Plug in the power cord to the wall outlet to start the string vibrating.
2. Hold the free end of the cord so that there’s a small amount of tension in the cord. Slowly
increase the tension by pulling the cord.
3. Predict whether increasing the tension increases or decreases the number of segments (“n”)?
Justify your prediction.
4. Check your prediction: does increasing the tension increase or decrease the number of
segments (“n”)?
5. Adjust the tension until there are exactly 4 segments.
(Be careful—the vibrating end is not always a node. You might need to make some adjustments
to make sure that the end is truly a node.)
Fix it so that it looks like a good node (even if you have to decrease the amplitude to do it).

How many wavelengths are on this string?

Measure the wavelength.

Calculate the speed of the wave.

What happens if one person touches an anti-node? Explain this observation in terms of
energy.

What happens if one person touches a node? Explain this observation in terms of energy.
6. Suppose that you have a string, fixed at both ends, that is being driven at 100 Hz by a device that
can only vibrate at that frequency. You observe that the resulting wave is not a resonance (ie,
no standing wave is produced.) Name two things you could do or change to produce a standing
wave.
Part 2
1. Detach the AC adaptor from the string vibrator and replace it with the sine wave generator,
as in this photo. Plug the power cord into the string vibrator.
2. Attach the free end of the string to a clamp on the far side of the table.
3.
Turn on the sine wave generator (there’s a switch on the side) and turn up the amplitude.
4. Adjust the frequency dial until you get resonance (ie, a nice standing wave.) If you get a
really loud annoying vibrating sound, that means the end is not a node—adjust the
frequency until the annoying sound goes away.

What mode (value of n) does this correspond to?

Find the the speed of the wave. Be careful with your units.

What is the fundamental (n = 1) frequency of this string?

Turn up the frequency until you reach the next mode. Record the frequency.

Find the difference between this frequency and your previous one. The difference between
consecutive resonant frequencies should be equal to the fundamental frequency. Is this
what you found?

Now go the opposite way—find a resonant frequency below the one you originally found.
Record the frequency.

Is the difference between this new frequency and the original one equal to the fundamental
frequency?
CONCLUSION
7. Explain why for some values of the tension there are standing waves with nice large amplitudes,
whereas other values of the tension produce junky waves. (Hint: what happens if the string is
being driven at a frequency that is not an integer multiple of v/2L?)
When you are finished, disconnect the sine wave generator and unclamp the string so that one
end is free again.
Make sure the instructor checks off your work before leaving the classroom.