If I understand the phenomenon being described, it is a bit like a ratchet/pawl operation. When string has sideways displacement it
is longer than when it is straight. Part of the extra length comes from stretching and part comes from the portion between the
pulley and mass, thus lifting the mass and moving the pulley. Ordinarily the pulley would move back and forth. However, if the
string slips over the pulley a little when the wave displacement is increasing, but it does not slip when the wave displacement is
decreasing, the pulley would rotate in the direction described.
I can imagine this could happen under some frequencies and tensions and not others. I could even imagine an interplay between the
wave on the horizontal portion of string and vibrations on the string between the pulley and mass. Thus, the length of string
between the pulley and mass might play a role in determining the conditions under which this happens.
Also, the elastic cord we use for this experiment stretches quite a bit, and gets thinner when it stretches. I could imagine it
would be inclined to slip more on the pulley when it stretches and thins compared to when it pulls back and thickens.
At any rate, any tendency to slip in one direction would create the ratchet/pawl action that would rotate the pulley.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Chair of Sciences
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.