Presentations

Feather and Dime

Falling in an evacuated tube at the same rate.

What it shows:

In the absence of air resistance all bodies, regardless of size or weight, fall with the same acceleration at the same point above the Earth. Here a feather and a dime (see Comments) fall under the influence of gravity in an environment where there is no air to mess things up.

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Bow and Arrow

Use conservation of energy to predict the height the arrow will reach.

bow and arrow

What it shows:

When the string of a bow and arrow is pulled from equilibrium, the elastic potential energy in the bow is converted to kinetic energy of the arrow when the string is released. When the arrow...

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Hula Hoop Rotational Inertia

What it shows:

A suspended hula hoop has the same period of oscillation as a pendulum whose length is equal to the diameter of the hoop.

How it works:

The parallel-axis theorem allows us to readily deduce the rotational inertia of a hoop about an axis that passes through its circumference and is given by

I = Icm+MR2 = 2MR2

where M is the mass of the hoop and R is its radius. The period of oscillation thus becomes T = 2∏√(I/mgd) = 2∏√(2R/g), which is equal to that of a pendulum whose length...

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Egg Shell Strength

What it shows:

Eggs have a reputation for being quite strong under compressional loads. You won't believe your eyes when you see how strong they actually are — an egg can support a person!

How it works:

An egg shell is a composite material, but primarily calcium carbonate, "nature's ceramic."1 We assume the ultimate compressional strength of the material to be about the same as bone (which is mostly calcium phosphate, but never mind): 170x106 N/m2. The diameter of a "large" chicken egg is about 1.75" and the typical shell...

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Vortex Tube

What it shows:

James Clerk Maxwell postulated that since heat involves the movement of molecules, it might be possible to separate hot and cold air in a device with the help of a "friendly demon" who would sort out and separate the fast and slow moving molecules of air. The vortex tube is such a device and does exactly that — using compressed air as a power source, it has no mechanical moving parts and produces hot air at one end and cold air at the other.

How it works:

Room temperature compressed air is supplied to the vortex tube...

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Chladni Plates

Accumulation of sand at nodes of vibrating plate reveals resonance patterns.

What It Shows

A Chladni plate consists of a flat sheet of metal, usually circular or square, mounted on a central stalk to a sturdy base. When the plate is oscillating in a particular mode of vibration, the nodes and antinodes that are set up form complex but symmetrical patterns over its surface. The positions of these nodes and antinodes can be seen by sprinkling sand upon the plates; the sand will vibrate away from the antinodes and gather at the nodes.

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Double Sound Source Interference

What it shows:

Two loudspeakers, separated about 1.7 meters emit the same tone of frequency 500 Hz and produce a pattern of constructive and destructive interference.

How it works:

At this frequency, the successive positions of constructive interference (maximum intensities of sound) occur approximately every two meters at a distance of 10 meters (which is roughly the middle of the lecture hall). The separation of maxima would be about 2.3 meters at 440 Hz. One way to make the interference pattern evident to the students is to...

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Musical Bottle

A beer bottle becomes a Helmholtz resonator when air is blown across its mouth.

musical bottle

Energy Stored in a Capacitor

What it shows:

The electrical energy stored in a capacitor is converted to mechanical work, driving a motor and raising a weight.

How it works:

A motor 1 is mounted atop a 2.5m length of 2×4. As it turns, it raises a 1 lb mass on a string from the ground by wrapping the string around a spindle (figure 1). The motor is driven by the discharge of a 12800µF, 75V capacitor previously charged by a DC power supply. 2 A double throw switch allows a clean change-over from one circuit to the other.
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Oersted's Experiment

What it shows:

Oersted showed that an electric current produces a magnetic field. His experiment is repeated here on a suitable grand scale.

Oersted's Experiment

How it works:

The current carrying wire in this case is a tubular 22mm diameter copper...

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