Demos: 1S-18 Turntable Oscillator

This excellent mechanical model illustrates a number of properties of waves and wave addition. The uniformly rotating turntables generate SHM, showing how the latter is a projection of the former onto an axis. Coupling the turntables generates a variety of wave forms that are simple combinations of sine waves. All of this is recorded by a felt tip pen on a moving sheet of paper.

Directions: Note carefully how the electrical connections are made. This will affect the order of switches to be activated. (See lecture demo technician for help.) Place the pen in the holder (with cap in place) and set it onto the paper. Turn the left turntable by hand and note the maximum swings of the pen. If it moves outside the range of the paper, readjust the post by moving it closer to the axis of the turntable. Remove the pen cap turn on the paper roll feeder. Then turn on the left turntable (set it at, say, 45 rpm) and note how the circular motion of the turntable causes the moving plate to execute simple harmonic motion. Before turning on the right turntable, stop the left one, turn off the paper feeder, and rotate each turntable manually to see that the extremes of swing don’t go off the paper. When this has been done, you can set several different combinations:

(a) To illustrate beats, set each turntable to the same speed setting (78 rpm or 45 rpm is best) and start the paper feeder. Then turn on both turntables. The weight of the paper feeder on the right turntable will cause it to have a slightly lower rpm. A beautiful pattern of beats will appear on the tape.

(b) Set each turntable at a different speed. This time you will get a complicated wave pattern that is the superposition of the two frequencies.

Suggestions for Presentation: The beauty of this demonstration is in its simplicity and transparency. For example, when illustrating superposition (beats or ratios), the students can see when the two systems are in and out of phase. Point this out as the turntables are operating. Try combinations such as 78:45, 78:33, etc, to see how the patterns change. We suggest that you do this in parallel with the oscilloscope readout of beats so that they can see that the same patterns emerge.

Applications: This sets the stage for a variety of superposition phenomena that occur in the unit on sound.

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Last Updated: May 9, 2016 11:44 AM