Institution: Washington
College
Address: Department
of Physics
300
Washington Avenue
Chestertown,
MD 21620
Apparatus Title:
Physical Pendulum with Self-synchronous Drive
Abstract (50-75 words)
A physical pendulum, with a strong magnet at the end, is pivoted between the arms of a two-pronged fork. The pivot is driven vertically by an electromagnetic wave driver to which the fork handle is attached. Pendulum passage is sensed by a Hall-effect device that can be positioned anywhere along the arc of swing. Signal-conditioning electronics permits application of an amplified pulse to the driver during either upward or downward passage of the pendulum.
Description:
This apparatus is designed to permit experiments on a pendulum that is driven by oscillating its support in a vertical direction. The driving signal is applied when the pendulum passes a chosen position along its swing. This ensures that the pendulum is always driven at its own frequency, even as this frequency changes with changing amplitude of oscillation.
The apparatus (see Figure below) consists of a physical pendulum made of a metal strip with a strong permanent magnet at its end. The pendulum is supported on a pivot between the arms of a two-pronged fork large enough to permit the pendulum to turn full circle. The fork is positioned in an “inverted-Y” orientation, with its handle attached to an electromagnetic wave driver.
A Hall-effect device is mounted on a carrier that can be positioned anywhere along a circular track. The track is in the same plane in which the pendulum swings and is fixed to the fork so that it remains concentric with the pivot.
The Hall-effect device produces a voltage output proportional to the magnetic field of the passing magnet, thus sensing the passage of the pendulum. This voltage changes polarity from positive to negative when the pendulum passes one way, and from negative to positive when it goes the other way. The support electronics can be set to produce a single pulse during either of these passages. This pulse is amplified and applied to the electromagnetic wave driver.
The experimenter can vary all attributes of the impulse applied to the pendulum pivot:
· The timing of the impulse in relation to the pendulum swing can be varied by changing the position of the Hall-effect sensor.
· The magnitude of the impulse can be adjusted by varying the amplification.
· The direction of the impulse can be changed by reversing the connections to the wave driver.