Vibration, a to-and-fro movement. The swinging of a pendulum, the quivering of a violin string, and the cycling of an alternating electric current are examples of vibrations. The energy of vibrations is transferred by waves, which in turn can produce new vibrations. Light, heat, radio signals, and sound are produced by vibrations.

A single vibration is called a cycle. The number of cycles made in a second is the frequency of vibration. All things capable of vibrating have a natural frequency at which they vibrate most easily.

Two objects that vibrate with the same frequency are said to be in resonance. If two such objects are brought near to each other, the vibrations of one will be transmitted to the other. When a vibrating tuning fork is placed near another with the same frequency, both give off sound waves. This is called sympathetic vibration. When two pendulums having the same frequency of vibration are suspended from a taut cord, the vibrations of one are transmitted to the other. This is mechanical resonance.

Piano tuners make use of tuning forks and resonance in tuning pianos. Resonance is important also in the production of musical sounds. A radio receiver picks up waves from a broadcasting station when it is tuned to resonance with the vibrations from that station. Resonance with the wind has destroyed great bridges, including the first Tacoma Narrows suspension bridge in Washington state. A column of soldiers marching in step can produce dangerous resonance in a bridge.

An object may be forced to vibrate at a frequency other than its natural one. When the stem of a vibrating tuning fork is touched to a table, for example, the table will vibrate at the same frequency as the fork. The table's vibrations will be transmitted to the fork, which will then vibrate with greater amplitude and give off a louder note. Forced vibration is used in stringed instruments. In a violin, for example, the vibrations of the strings are transferred to the wood and transmitted to the air within the instrument.

The control of destructive vibrations is important in engineering. Vibrations would destroy an improperly designed airplane that tried to exceed the speed of sound. Springs, shock absorbers, and tires are used to reduce vibrations in automobiles. The flywheel and counterweights on the crankshaft reduce the vibrations of an automobile engine.