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When I was actively teaching I would demonstrate the Doppler Effect (and several other things) with the aid of a common physics laboratory support rod made of an  aluminum alloy and about a meter long.  The rod is held between two fingers at its exact midpoint and one end of the rod is tapped against the floor or with a block of wood.  The direction of the tap should be along the length of the rod rather than crosswise so that the rod vibrates longitudinally rather than transversely. (Any transverse vibration, which is audible as a lower pitch, can be suppressed by gently and quickly brushing one end of the rod  between thumb and finger.)The rod can be heard ringing at a rather high pitch and if held at the exact midpoint (a node) it will continue to ring audibly for a minute or more.  Pinching one end of the rod (or any place which is not a node of the standing wave pattern) damps the vibration almost instantly. Different length rods will give different pitches (frequencies). 

Doppler Demonstration: If the ringing rod is pointed towards the audience and momentarily quickly moved towards them and away from them the raising and lowering of pitch due to the motion of the source is clearly audible and easily related to the observed motion of the rod.

Also, since sound radiates from both ends of the rod and the wavelength of the sound in air is rather short, the rod can be slowly rotated about its midpoint and the individual audience members can hear the interference pattern sweep past them.

Furthermore, you can set the rod vibrating at the next harmonic, an octave above the fundamental, by holding the rod at its exact quarter length from either end.

An electronic alternative for showing the Doppler effect involves  a high-pitched audio oscillator from Radio Shack for a dollar or two, powered by a small dry cell and swung in a circle at the end of a couple of feet of lamp cord connecting the battery holder (held in hand) to the oscillator.

Al Read
Science Discovery Center of Oneonta (NY)


-----Original Message-----
From:   Informal Science Education Network on behalf of Marc Taylor
Sent:   Wed 8/11/2004 10:39 AM
To:     [log in to unmask]
Cc:	
Subject:        doppler shift demo

ISEN-ASTC-L is a service of the Association of Science-Technology Centers
Incorporated, a worldwide network of science museums and related institutions.
*****************************************************************************

for a workshop about finding extrasolar planets, I am looking for a convenient way to demonstrate Doppler shift. (An intern of mine told me how cool it was when her science teacher demonstrated it to them by driving by the school with his horn blaring -- not feasible, sorry)

The obvious thing is some sort of constant-tone noisemaker whirled around on a string - but in a small (10m/33ft) room, and with a lot of elderly docents running things, this doesn't seem like a good idea.

Of course, I could depend on their experience, but to have a concrete example would be best. Any suggestions?

Marc Taylor
Coordinator, Andrus Planetarium
Hudson River Museum
511 Warburton Avenue
Yonkers, NY 10701
(914) 963-4550 x223
Fax:  963-8558
[log in to unmask]

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