The tuning fork forces surrounding glass (or vinyl) particles into vibrational motion. However, if the tuning fork is set upon the whiteboard panel or the glass panel of the overhead projector, the panel begins vibrating at the same natural frequency of the tuning fork. The sound produced by the tuning fork is barely audible to students in the back rows of the room. If the tuning fork is held in your hand and hit with a rubber mallet, a sound is produced as the tines of the tuning fork set surrounding air particles into vibrational motion. This same principle of a forced vibration is often demonstrated in a Physics classroom using a tuning fork. This causes an increase in the amplitude and thus loudness of the sound. In the case of the guitar string mounted to the sound box, the fact that the surface area of the sound box is greater than the surface area of the string means that more surrounding air particles will be forced into vibration. The tendency of one object to force another adjoining or interconnected object into vibrational motion is referred to as a forced vibration. The entire system (string, guitar, and enclosed air) begins vibrating and forces surrounding air particles into vibrational motion. The sound box in turn forces air particles inside the box into vibrational motion at the same natural frequency as the string. On the other hand, if the string is attached to the sound box of the guitar, the vibrating string is capable of forcing the sound box into vibrating at that same natural frequency. If you were to take a guitar string and stretch it to a given length and a given tightness and have a friend pluck it, you would hear a noise but the noise would not even be close in comparison to the loudness produced by an acoustic guitar. This input of energy disturbs the particles and forces the object into vibrational motion - at its natural frequency. Whatever the case, a person or thing puts energy into the instrument by direct contact with it. For instance, a guitar string is strummed or plucked a piano string is hit with a hammer when a pedal is played and the tines of a tuning fork are hit with a rubber mallet. Musical instruments and other objects are set into vibration at their natural frequency when a person hits, strikes, strums, plucks or somehow disturbs the object.
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