PHY 2048L
Lab 9

Standing Waves on a String

Remember to hit the "reload/refresh" button to see changes made to this page.

Updated: 8/23/2008
==============

Note: The lab manual uses "rho" for mass per unit length, whereas we use "mu".
You should change the equations to reflect the ones we use in class.

Prelab

(1) Answer all 7 questions on pages 221 and 222 of the lab manual.

(2) Read the procedure handout:
The procedure for this lab is changed significantly from that in the lab manual. See comment below for the reason for this change. As described in the handout for this lab,

we will keep the wavelength constant and vary the frequency as we vary the tension. (This requires a significant change in the data tables, so you will also get a handout with a revised data table for this lab.) Read through the procedure handout and use it to make notes in your lab manual saying what will be done.

(3) Ignore the data table on page 223.
We will use a modified data table (32 kB pdf).

(4) Change the post-lab questions:
See the handout for the editorial corrections. Some are trivial (replace wavelength with frequency, or vice versa), but some require care. Ask questions before you leave the lab if what you need to do is not clear.

Comment

The lab description in the manual assumes you have a fixed frequency source (at the 60 Hz line frequency) and thus varies the tension to get different numbers of nodes. Because we have a very accurate way of adjusting the frequency, the procedure for this lab will be essentially the reverse of the one described in the manual: We will keep the number of nodes and the wavelength constant, and find the frequency required to produce that standing wave for a range of tensions. See handout for details.

The mass per unit length of the string will be measured at the start of the first lab. It should be about 0.15 g/m (less if we used the thinner string).

We will vary the mass used to put the string under tension and measure the frequency required to produce three antinodes (the n=3 mode) on the string. The slope of the fitted line will then give us an estimate of the wavelength that can be compared to the wavelength determined from a measurement of the length of the string between the driver and the pulley.

Apparatus

pulley, backdrop, driving motor, and wave generator

Blue backdrop makes the string (not shown in this picture) visible. String goes over the pulley at left and is attached to the driving motor (actually a speaker) at right with frequency set by the wave generator (see closeup below).

connections for wave generator (bottom) and frequency counter

Closeup of coax cable connections for wave generator (bottom) and frequency counter plus 'tee' and splitter for feed to speaker. The wave generator controls both the frequency (knob at far right) and amplitude (knob just to the left of the coax) of the wave while the frequency counter gives an accurate reading of the frequency.

Be sure the "sine wave" button is pushed in. This equipment is used for several labs in PHY2049, and the shape of the wave is varied in one of them.

 
==============

?? Contact me if you have any questions.