Al Tobias (wat4y) - Office: Gibson S123 & Physics 218, (434) 924-0538

# Physics Demo Manual

Demonstrations are cataloged according to PIRA Bibliography

Due to Physics Building renovations, the lead time to set up demo requests has increased due to the need to transport equipment across campus. Please be kind and let me know well ahead of time what you need.

 Choose a Topic or Enter a keyword to search: I cannot find what I want! You have selected the following Demos: None Selected

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Harmonic Motion
Waves
video  - Transverse Waves
video  - Longitudinal Waves
Acoustic levitation
video  - Standing Waves
Soap Film Standing Waves
Bell in a Vacuum
video  - Doppler Buzzer
video  - Diffraction & Interference in Ripple Tank
Double Slit Interference of Microwaves
Interference of Sound Waves
video  - Tuning Fork Beats
Theremin
video  - Oscillator Beats
Acoustics
Instruments

### video  - Standing Waves

#### Purpose:

To produce standing waves in a string and demonstrate the dependence of the resonant mode frequencies on the tension in the string.

#### Procedure:

We have two setups, the older reciprocating saw and the mechanical vibrator with function generator.

RECIPROCATING SAW: (1st & 2nd images below) Clamp motor base to lecture bench on one end and the table clamp with support ring to the other end. Position support ring so that the string runs from the motor cam, across the bench, through the support ring, and hangs over the edge of the table. Attach weights to the string so there is room for them to move freely. Start the oscillator slowly. The strobe is triggered by the driving frequency of the motor. Slowly increase the motor speed until a standing wave resonance is found. It will be possible to see the first three resonances before reaching the limit of the rotation rate of the motor and maybe more depending on the weight used. For a more quantitative analysis, the resonant frequencies of identical modes at different tensions (weights on the string) can be read from the strobe and compared with theory (the ratio of two different tensions should be equal to the squared ratio of the resonant frequencies for the same vibrational mode).

MECHANICAL VIBRATOR W/ FUNCTION GENERATOR: (3rd & 4th images below) Although a smaller setup, this is more precise and the fundamental frequencies can be read off the generator. One can add enough mass on the end of the string to adjust the frequencies such that they show up at integral values in Hz.

#### Hints:

For the reciprocating saw setup, adjust the weights/tension (approximately 2kg) and the length of the spring such that the fundamental happens above 35V on the VARIAC setting, otherwise the power tool has difficulty maintaining a steady motion and the fundamental frequency gets ruined after a short time.

For the mechanical vibrator w/ function generator setup, you'll need a mass of about 200g. Add small units of mass until you achieve frequencies at integral values.

An alternative set up with two vibrators is available (5th image below). Using this setup one can demonstrate the polarization (linear if ran from the same generator and circular/elliptical if ran from different generators). The strobe delay can be adjusted using the strobe delay box used in the soap film standing waves set up.

#### Equipment:

• Strobe Light
• C-clamp
• Motor driven oscillator
• String
• Support Ring
• Table Clamp
• Weights