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Attendance was light, not surprising given the summer break, but it was an excellent proof of principle. Brand new AAPT member Chandra Wu fresh from her first year at the Seattle Public schools stopped by as did Libby Schoene of South Seattle College, Kevin Wheelock of Bellevue College and your Section Representative (and faithful reporter) Robert Hobbs of Bellevue College. The entire Green River contingent begged off with thin excuses such as being in Greece, India and other inconvenient locations. However we were joined by Kurt and Barbara Hoffman from Whittman College. They had a trip planned to the area already and seen the message just before they left Walla Walla. Later in the evening some of the Bounty staff had friends stop by and we amazed them with a few of the demonstrations.
As people were arriving, Robert and Kurt had a few things already laid out. Robert showed off some surprising eddy current effects achievable with the strong Neodymium magnets now available. One set employed a large slab of ½ inch thick copper plate. A large disk magnet tossed spinning onto the slab would slow and settle without a sound losing most of its velocity in the last half centimeter or so, the expected slap of the disk face against the copper never materializing. A cylindrical magnet would roll leisurely down the plate when inclined, never picking up the expected speed for typical g sin(theta) rolling. It also automatically steered away from the edges. He also brought his variation of the magnet down the tube demo in the form of a set of copper pipes of varying wall thickness and one pipe with longitudinal cuts spaced around the walls. THe magnet was seen to fall more slowly through the thicker wall pipe and since the cuts interrupt the circumferential currents, the magnet falls rapidly through the pipe with the cuts.
Kurt showed us two very nice driven oscillator resonance demos. One simple, using a dowel rod and small weights (bundles of paper clips) hung with different length strings. When the ends of the rod are held out using fingertips, you can drive the pendula with very small finger motions. It is not difficult to pick out the resonance for any desired pendulum making it swing with large amplitude as the others hold relatively still or make low amplitude beat responses. The other employed a home made set of resonance tines like a large comb of a music box. This was clamped between two lengths of “L” iron and when the iron base is driven by a string connected to a PASCO mechanical vibrator, individual tines are seen to resonate while their neighbors remained still or nearly still. This device models the human choclea in the ear where a membrane of reducing width and thickness resonates in different locations to sounds of different frequencies. A mechanical Fourier Transform device mapping frequency to location.
Robert followed with a favorite demonstration devised by Caleb Teel using asymmetrically weighted pendulum bobs that are part of a guided inquiry into the variables that affect the period of the pendulum. Caleb’s approach gets students to engage substantially with relationships in science and how they are established. Surprisingly half the class legitimately determine that mass plays a role in determining the period while the other half obtain compelling data that period is independent of mass. The asymmetrically weighted bobs are employed after this conflict is revealed in the community results. Two apparently identical pendula are drawn aside to the same angle and released. Shortly after the first period it is already becoming evident that one is falling behind the other. After a few periods there is clearly a changing phase relationship. As they continue the fall back in step after ten to fifteen cycles. This is very mysterious. Of course they are hung with the more massive end up for one bob and down for the other so that the length of the pendulum (to the CM point) is not the same. Caleb passes the bobs around and the students discover how the trick was created. This sets the stage for a discussion what does the length of a pendulum mean and what should it mean. For many students the question of what length did they measure is in doubt since they did not make a complete enough record to reliably recall. A discussion of what is important in the data record naturally follows.
Robert then displayed a wilburforce pendulum and as everyone watched energy exchange between the torsional oscillation anf the translational oscillation he noted that this was the demonstration on Watch Mr. Wizard that was largely responsible for his choosing to go into physics.
Kevin then shared some creative assignments that require the students to research and estimate reasonable values before they are able to make the calculatons. In one example students are asked what force and torque Iron Man applies in the avengers movie as he strives to restart the rotor on the dammaged SHIELD ship before it crashes.
Next we looked at Laser spots in mirrors. Some of you will remember Roberts challenge at the 2011 meeting at CWU concerning what will be seen when a laser is aimed at a mirror. Fortunately none of those in attendance last night had seen this one so after some relatively astute predictions we observed and discussed the result. Several people noted how satisfying it would be for students who work through the puzzle to not only explain the extra dots, but also the order of the brightnesses of the dots in the sequence.
Libby showed up during the mirror activity after a heroic drive straight through from Eugene (No she doesn’t live there.) After obtaining refreshment, she shared a very simple, effective and tasty(!) moment of inertia demonstration using toothpicks and Gumdrops. We ate it up!
At this point proceedings broke into conversational knots as participants followed individual interests. Libby caught up on the Demonstrations she missed, and some friends of Bounty employees came in from the patio to play with the ping pong balls in the air stream demonstration (Click image for video!). Robert put on his in-line skates and back pack with bathroom scale and had one of these guests push him along keeping the scale reading steady between 5 and 10 lbs. Sophia was surprised to discover that maintaining constant force requires increasing speed (Click image for video!). Meanwhile, Libby practiced with the centripetal force (water in a bucket) demonstration. The Bellevue College version is a water glass on a plant hanger tray (credits Kris Whelan). A very unstable appearing device that holds together very well through the whole cycle.