Presentations

Microscope Resolution Tuesday, December 6, 2016

What it shows:  The wave nature of light limits our ability to see the very small. Application of the Rayleigh limit of resolution tells us that the size of the smallest objects one can resolve under a microscope is approximately equal to the wavelength of light. The optical limits of a microscope are demonstrated as one attempts to resolve 1 μm diameter spheres (about twice the wavelength of light) — one sees spots of light surrounded by diffraction rings rather than sharply defined spheres, similar to the 3rd image (from: Cagnet/Francon/Thrierr, Atlas of Optical

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Air Table Center-of-Mass Motion Monday, May 2, 2016

What it shows:  Two bodies, rotating about each other, rotate about their common center-of-mass (COM). The COM exhibits uniform motion (or none at all) regardless of what the two bodies are doing.

How it works:  The "bodies" are 4-1/2" diameter acrylic disks that float on a cushion of air on a large air table.1 Presently we have three versions ready to go. (1) The first version has two disks connected by means of a 12"- long plastic ruler. A large "dot" at the center of the ruler marks the COM. The disks can be made to simply

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Vortex Shedding in Air Friday, April 8, 2016

A thin wire, moving through the air, is made to vibrate in the audio range at the vortex shedding frequency.

What it shows:  When air flows around an object, there is a range of flow velocities for which a von Karman vortex street is formed. The shedding of these vortices imparts a periodic force on the object. The force is quite small and not enough to accelerate the object to any significant amount, especially if the object is relatively massive. If the situation is such that the object can vibrate about a fixed position, we have the possibility of simple

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Pulse Reflections in a Coax Cable Thursday, February 25, 2016

What it shows:  A voltage pulse, injected into a long coaxial cable, will travel down the length of the cable and undergo a reflection at the other end. The nature of that reflection depends on how the cable is terminated at the other end. Shorting the cable at the far end produces an inverted reflection. With no termination (an "open" end), the reflected pulse is not inverted. When the impedance of the termination matches that of the cable, there is no reflection.

Knowing the length of the cable and noting the amount of time it takes the pulse to come back

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Reverse Sprinkler Friday, December 18, 2015:

What it Shows

Inspired by Richard Feynman's story in his 1985 book (pp 63-65), Surely You're Joking Mr. Feynman, the demonstration answers the question "which direction does a lawn sprinkler spin if water enters the nozzle rather than being expelled from the nozzle?" The reverse sprinkler spins in the opposite direction of a "normal" sprinkler. "Dissipative effects" has been the hand-waving reason for the past 30 years, but the real reason why it spins in the reverse direction is far from obvious (see Comments, below). It turns out that a sprinkler designed to be "truly

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Three-Legged Table

Triangular table supported by platform scales.

What it shows:

This is a two-dimensional version of the Loaded Beam demonstration.

How it works:

An equilateral triangular-shaped table is supported at each corner by a platform scale (same type as in "Loaded Beam"). One of our large (14.5 kg = 32 lb) weights is placed on the table. The scale readings vary with the position of the weight. The procedure to be followed in this

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Pendulum and Nose

Faith in the conservation of energy is tested by taking the demonstrator's nose to task.

What it shows:

The principle of conservation of energy ensures that a pendulum released at a particular amplitude will not exceed that amplitude on the return swing. A lecturer's faith in their subject is put to the test using a 50lb (22.7kg) iron ball.

How it works:

Technique is very important here. The best method to employ is to stand with your back against the blackboard with your head also touching the board. This ensures that you don't lean forward after

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Foucault Pendulum

Plane of pendulum oscillation appears to change due to rotation of Earth.

What it shows:

Due to the rotation of the Earth, the plane of oscillation of a pendulum will rotate with respect to the surface beneath it. We expect a rotation of about 10˚/hr at our latitude of 42.˚

How it works:

Here the observer standing on the Earth resides in the reference frame, with the swinging pendulum oscillating in a rotating frame. From the pendulum's point of view, it keeps oscillating in the same plane, but the Earth spins below it. The deflection from its original

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Shattering Wineglass

Large speaker with signal generator/amplifier destroys a wineglass; stroboscopic illumination shows vibration mode.

What it shows:

Sound waves of the right frequency are used to excite a wineglass in one or two of its normal modes of vibration. Stroboscopic illumination makes it possible to actually see the vibrations in apparent slow motion. When the intensity of the sound is increased, the large undulations of the glass exceed its elastic limit and cause it to shatter. This can be done in the fundamental or next higher normal mode of vibration ... a beautiful and

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Bell Plates

What it shows

Bell plates are polygonal-shaped flat pieces of sheet metal which, when held in the hand and struck with a beater, produce a pleasant, sustained, slightly bell-like tone. Compare this to any arbitrary shaped piece of metal which produces a "clunk" when struck. The sound of the bell plate depends strongly on its shape and even the most modest change in the symmetry (like snipping off a corner) or proportions will make it go clunk when struck.

How it works

Why does a particular shape ring so well,

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