A small styrofoam (or pith) ball on a thread is held next to a vibrating 512-C tuning fork. You can’t see the tuning fork’s movement, but the styrofoam ball responds. This is a demonstration to prove that a vibrating body can capable of producing sound, a Science Sir video.
Slow down the day’s pace and observe as each element of the experiment is introduced. The first experiment starts at 1m59s. The second experiment, the amplification of sound as the tuning fork touches the table, starts at 3 minutes. A quick definition of sound from Wikipedia:
In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.
In human physiology and psychology, sound is the reception of such waves and their perception by the brain. Only acoustic waves that have frequencies lying between about 20 Hz and 20 kHz elicit an auditory percept in humans. Sound waves above 20 kHz are known as ultrasound and are not audible to humans. Sound waves below 20 Hz are known as infrasound. Different animal species have varying hearing ranges.
Here’s another version of the first experiment: Tuning Forks Resonance + Ping Pong Ball, this time with two tuning forks that have matching frequencies. As the first tuning fork is struck, sympathetic resonance sets the second tuning fork and its neighboring ping pong ball(?) in motion.
Read more about tuning forks at How Stuff Works. Then watch this: Resonance, forced vibration, and a tuning forks demo.
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