Gas station without pumps

2011 December 4

Slinky lab

Filed under: home school — gasstationwithoutpumps @ 20:54
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In the Physics homework (Chapter 4) post, I mentioned one lab I wanted to do:

Compression waves in a Slinky.  We want to measure stiffness and mass as well as the speed of compression waves.  That way we can compare our simulation to real measurements.

Well, my wife was downtown today and bought us a Slinky in the toy store, so the lab is on for this Friday.  Here are some things we should do:

  • Determine the mass of the Slinky.
  • Determine the number of turns of the helical coil (and hence get mass per turn).
  • Determine the stiffness of the spring.  Since the Slinky has a low stiffness and a fairly high mass, it might be good to do this by measuring the stretch of the slinky under its own weight.  Note that the top of the Slinky stretches much more than the bottom, so it is necessary to come up with the proper model for how a spring stretches under its own weight (a fairly simple thing, but a good test of understanding).
  • Come up with some other way of measuring the stiffness of the Slinky, and compare it with the self-weight estimate.
  • Estimate the stiffness for each turn of the Slinky, so that we can model it as a stack of many identical 1-turn springs.
  • Write a Vpython 1D simulation of the Slinky, using the mass and stiffness of each turn of the helix a separate object.  (This is essentially 4.P.90, but using measured mass and spring constants, rather than estimates of atomic masses and metal-lattice bond stiffness.)
  • Videotape compression waves in the spring, and estimate their speed (the “speed of sound” in the spring).
  • Compare the measured speed of sound with the Vpython computation.

If the 1D simulation does a good job, we can look into doing a full 3D simulation of a Slinky, and see if we can simulate it walking down stairs.

Unfortunately, I’m probably going to have to use the same household computer than my son uses for the Vpython programming.  I had to break Vpython on my  laptop, so that I could run a different Python package that I needed for work.  Unfortunately, Vpython only runs on the obsolete 32-bit Python, because it is tied to the obsolete Carbon framework on the Mac.  This means that Vpython will be dead within 5 years, unless the developers can figure out how to run it under Cocoa (they claim there is a thread priority problem in Cocoa that they don’t know how to work around), or go back to running under X Windows.  Actually, there is an X Windows version of Vpython that supposedly can be compiled for the Mac, but I read the installation instructions for it, and decided that Vpython was simply not worth that much hassle—if I wanted to spend all my time figuring out arcane installation techniques I would have bought a Linux box, not a Mac..  If they rewrote the Mac installer so that you had the choice of 32-bit Python and Aqua windows or 64-bit Python and X windows, I’d do it.


  1. […] 4) I proposed 3 labs to do with Chapter 4 of Matter and Interactions.  Last week we did the Slinky Lab (though students have not done the computational simulation of the Slinky yet and have not tried […]

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