Fluctuations is movement, that on or about repeated intervals of time. A group of bodies, the movement which is studied in mechanics, called a system of bodies or just the system. Power, acting between bodies of the system, called internal. External forces are called forces, acting on a system of bodies, non.

Kolebaniya, arising in the system under the action of internal forces are called free vibrations. ( For removing the system from the equilibrium position of the external driving force is applied one or more times) Fluctuations, performed by the bodies under the action of periodically varying external forces, called internally.

Pendulum – a material point suspended by a weightless inextensible thread.

Harmonic oscillations – periodic change of a physical quantity depending on time according to the law of sine or cosine. The function cosine is applied, when the body starts oscillating motion from maximum position. Function sinus is applied, when the body moves from the equilibrium position.

The amplitude of the (And;Hmm;X0 m.) – the maximum deviation of the body from the equilibrium position.

Period (T*S.) - time, for which the body performs one full oscillation. For one full oscillation of the body covers a distance of four amplitudes.

Frequency ( Hz=1/s) – the number of oscillations, committed by a body per unit time.

The phase of the oscillation ( happy.- radian ) – all that stands under the sign of the sine or cosine. The phase of the oscillation determines when a given amplitude of the oscillatory state of the system at any time.

Resonance – sharp increase in amplitude of forced vibrations at coincidence of frequency change of the external force, operating system, with the frequency of free oscillations of the system. The less friction in the system, the greater the amplitude of the resulting oscillations. Undamped oscillations, which may exist in the system without exposure to external periodic forces, called self-oscillations.