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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Cavendish Experiment

Calculation of gravitational constant, with accompanying apparatus model.

What it shows:

The gravitational attraction between lead spheres. The data from the demonstration can also be used to calculate the universal gravitational constant G.

gravitational attraction

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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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Doppler Turntable

Two speakers, one at each end of rotating platform; beating due to frequency shift of speakers travelling in opposite directions.

What it shows:

Doppler shifting of sound to higher frequencies occurs when a source is moving towards the observer, and shifted to lower frequencies when the source is moving away. Here two sources emitting the same frequency when stationary rotate on a turntable. With one source moving towards you and one away, the Doppler shifted waves interfere to create beats.

How it works:

Two 1.5W 8Ω...

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Collisional Broadening

What it shows:

Perturbation by colliding atoms in a high pressure gas result in the broadening of emission and absorption lines. This is clearly seen in the sodium D (589nm and 589.6nm) lines of a high pressure sodium lamp.

The broadening in frequency width is dependent upon the separation of the perturbing particles (Novotny 1973) by

∆ν ∝ r-n

With n=2 the broadening is due to the coulomb field of an ionized atom or electron; this is the linear Stark effect. With n=3 the interaction is between neutral atoms of the same type; this...

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Reaction on Conveyor Track

What it shows:

A straightforward demonstration of Newton's 3rd law, that forces are interactions and thus come in pairs.

How it works:

Two people, each sitting (cross-legged) on their own board, position themselves in the center of the track facing each other. Upon pushing against each other with their hands, they glide apart down the length of the track. Repeat this with one person turned around — the other person pushes on his/her back instead of pushing against each other with their hands. The ensuing motion down the track is exactly the same as before.

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Collapsible Cow

What it shows:

A small crude spindly model of a cow is able to support five times its own weight. Another model, scaled up exactly six times in all dimensions, collapses under its own weight! Assuming that strength is proportional to cross-sectional area (∝ dimension 2) and weight is proportional to volume (∝ dimension 3), simply scaling the model up geometrically leads to the situation where the weight is too great for its strength.

How it works:

This demonstration was inspired by R.H. Stinson's apparatus note in the AJP (see References...

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