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Concerning model seismographs....
Over the past few years I have had several Elementary Education students try to set up a seismograph-like device as an activity to present to our Saturday Afternoon visitors (a Science Education course assignment to be done in our Science Discovery Center, State College, Oneonta, NY). All have had only modest success.
As I understand seismographs, they depend on the inertia of a rather massive object suspended from a compliant support attached to the "earth." If there is a shaking back-and-forth of "earth" and the support holding the mass, the mass remains at rest (due to its inertia) while the "earth" moves horizontally back and forth underneath it. For the mass, this is a direct application of Newton's First Law of Motion, although some would point out that it also illustrates Newton's Second Law since the horizontal forces on the mass by the strings are very small as the supports wiggle back and forth, so any horizontal acceleration of the mass will be correspondingly small and can be neglected.)
A typical setup uses a 500 gram mass suspended by a pair of half-meter (or longer) strings. The strings are attached to one point on the mass and go up in a V shaped manner to two points 5 or 10 cm apart on a horizontal support rod. This arrangement restricts the motion of the mass relative to the rod to a left-right direction perpendicular to the rod. The horizontal rod is connected to a vertical rod clamped to a small shaky table whose top is a few cm. below the bottom of the 500 gram mass. A short tube is found through which a felt-tip pen can slide freely but snugly, and this tube is attached vertically to the mass so the pen can leave a mark on a sheet of paper resting on the shaky-table's top with a negligible dragging force. If the paper is pulled slowly forward (in a direction parallel to the horizontal rod holding the V-shaped strings) and the table is tapped sidewards (in a horizontal, left-right direction perpendicular to the horizontal rod) the table and paper will move left and right under the stationary mass and a seismograph-like trace will be left on the paper.
A fancier setup would amplify the relative motion of the mass and the tabletop by a horizontal lever arrangement. A rather long, rigid but very light rod is pivoted at one end by an attachment to the table, and a rather short distance from the pivot there is a pin which goes through the rod vertically upward into the bottom of the suspended mass. A pen is affixed to the outer end of the rod. The amount of magnification of the relative movement is estimated by the ratio of the pivot-to-pen distance to the pivot-to-mass distance. This works to amplify the relative motion only if the drag force of the pen on the paper is negligibly small. Otherwise, the pen-paper drag force is amplified by the same lever to produce a horizontal force on the mass causing it to move along with the paper and defeating the original premise underlying its use....that the mass remain at rest while the "earth" shakes underneath it.
My apology for being so wordy in this response. I hope it may be helpful to anyone persistent enough to read through it. (Old teachers never die, they just ramble on.)
Al Read, Science Discovery Center of Oneonta (NY)
-----Original Message-----
From: Informal Science Education Network on behalf of Natasha Aristov
Sent: Sat 11/15/2003 9:37 AM
To: [log in to unmask]
Cc:
Subject: home-made seismograph
ISEN-ASTC-L is a service of the Association of Science-Technology Centers
Incorporated, a worldwide network of science museums and related institutions.
*****************************************************************************
Howdy fellow listers,
would you folks have designs for a "make-and-take" seismograph that kids
could put together? We're devising an earth science hands-on exhibit for
our school's museum.
Thanks for your help!
Natasha
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