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Tension Puzzler

What It Shows

The two ends of a dial-type spring balance are each connected to strings which run over pulleys. With equal weights attached to the ends of the strings, the spring balance indicates the value of one of the weights.

How It Works

The demonstration is presented to the class as a puzzler: the spring balance is turned around so that the class can't see the dial. Students are invited to guess what it is reading. Invariably they guess the sum of the two weights. The lecturer then turns the face of the dial gauge around showing them the error of...

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Tennis Racquet Flip

What it shows:

A simple and convincing demonstration of the intermediate axis theorem. Consider an object (a tennis racquet in this case) with three unequal principle moments of inertia. If the racquet is set into rotation about either the axis of greatest moment or least moment and is thereafter subject to no external torques, the resulting motion is stable. However, rotation about the axis of intermediate principle moment of inertia is unstable — the smallest perturbation grows and the rotation axis does not remain close to the initial axis of rotation.

How it works:...

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Slinky Wave Cradle

Longitudinal wave demo with suspended slinky.

What it shows:

Demonstration of longitudinal traveling waves in a spring. 

How it works:

The Slinky hangs with a bifilar suspension from a rigid thin-walled electrical conduit frame, which is light, strong and cheap. In total, 23 suspension points run the length of the spring; the cord is a thick cotton thread that attaches to a loop of the Slinky with No.10 fishing swivels. The layout of the Slinky and frame are shown in figure 1, but the thread has been...

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Conservation of Charge to 2 Sig Figs

What is shows:

A neutral system of charges is rearranged...charge measured on one part is equal and opposite to the charge on another part. In that respect, this demonstration is not much different from the " 3 Sig Figs" demo in which voltage measurements are used. Conservation of charge is typically introduced in the first few lectures of an E&M course, before the concepts of voltage and capacitance are discussed. If voltage is the quantity...

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Magnetic Levitation

What it shows:

A magnet tethered over a spinning aluminum disc levitates due to induced currents in the disc.

How it works:

As the disc spins, electrical currents are induced in the aluminum as it moves with respect to the magnet. These induced currents create a magnetic field which, in accordance with Lenz's law, opposes the field of the magnet. The magnetic repulsion causes the rider to levitate about 1cm above the disc. Lenz's law also says that the induced field will oppose the motion that causes it. The magnet therefore tugs...

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Disappearing Prism

What it shows:

Light is refracted as it passes between two transparent materials of different refractive indices. If the materials are different, but the refractive indices are not, then the light rays are undeviated and the materials are optically indistinguishable.

How it works:

"And if you put a sheet of common white glass in water, still more if you put it in some denser liquid than water, it will vanish almost all together, because the light passing from water to glass is only slightly refracted or reflected or indeed...

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Resonance Radiation/Absorption

What it shows:

For an electron to make a transition from one energy level to a higher one, it needs to absorb a photon who's energy is equal to the difference in the energy levels involved. When jumping back down, it will emit a photon of that same energy. These discrete energy separations are characteristic of the atom involved, and it's what provides an atom with its fingerprint line spectrum. Trying to induce a transition with a photon of different energy just doesn't work.

In this demonstration, light from a sodium source will be absorbed by sodium gas...

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Spherical Blackboard

What it shows:

You can use a spherical blackboard for many things, including the teaching of geographical coordinates, as a model for a closed Universe, or simply as a mathematical shape.

In the non-Euclidean geometry of the sphere, a circle will have a circumference greater than 2πr and an area greater than πr2. A triangle’s angles will add to more than 180°, and two parallel lines, called Great Circles, will converge.

A Universe with a density parameter Ω greater than unity will have too much mass to overcome its own gravitational...

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Armillary Sphere

Model to show celestial sphere; larger version has capacity to show lunar motions.

What it shows:

The position and motions of heavenly bodies are projected against a hypothetical sphere of infinite radius, centered on the Earth, called the Celestial Sphere. With this demo you can explain the motions of the stars and of the Sun, and show various aspects of the seasons.

How it works:

The main features of the sphere itself are shown schematically in figure 1. The spherical wire cage defines the celestial sphere, its...

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Nitrogen Phase Change

Liquid nitrogen is pumped on and freezes into a sponge of solid nitrogen.

The liquid nitrogen is in a 600 or 800 ml beaker under a shielded bell jar on top of the red vacuum cart. A cold trap is not necessary if only nitrogen is being pumped on.

It is important that the beaker of liquid nitrogen not have frozen water vapor on its side, as the view is impaired. A camera is zoomed in on the beaker, which is in a thick glass bell jar and an acrylic tube shield.

With the pump running and the bell jar vent open, pour the nitrogen and cover the beaker with bell jar. Open up...

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