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The bodies, surrounding us, are moving, interact with each other, and some bodies are motionless. The part of mechanics, which investigates the conditions, at which the body is motionless though at presence of external actions, is called the statics. The statics, as the science which has arisen in a remote antiquity and developed under influencing of practical needs of mankind, connected with building of different ground-level facilities, bridges, ships.

Before to speak about conditions of a body equilibrium it is necessary to agree upon what is meant equilibrium, otherwise there can be misunderstanding.Let's suppose that a body is in the equilibrium in some frame of reference, if in this frame of reference a body is at rest, that is that all macroscopic parts of a body are motionless. Let's adduce examples of the bodies in the equilibrium.

A bookshelf, which is suspended on a wall in a room, is in the equilibrium relative to the inertial frame of reference, which is connected with this room. A chair, which stands at a rotating stage of a theatre, is in the equilibrium relative to the non-inertial frame of reference, which is connected with this stage. A suitcase, which stands on the floor of a moving wagon, is in the equilibrium relative to the frame of reference, which is connected with this wagon. If the wagon moves rectilinearly and uniformly, then this frame of reference is inertial, and if the wagon moves with an acceleration, then this frame of reference is non-inertial.

The statics is the particular case of the dynamics, when all velocities are equal to zero. Therefore all conditions, which determine the equilibrium of a body, may be obtained from laws of the dynamics, and, consequently, from Newton's laws, which constitute the basis of the dynamics. Later, during the investigation of the conditions of the equilibrium of a body, we will limit ourselves by the consideration of the equilibrium conditions only in inertial frames of reference, because we already learned the laws of dynamics in these frames.

In the next section we will make more accurate study of that, how we should work with forces, and what is the resultant force. After that we will discuss particular cases of the equilibrium of a body and the general case.


§1. Introduction. §2. Force. Equivalence of forces. Resultant force. Addition of forces. Decomposition of force. §3. Equilibrium of a material point. §4. Equilibrium of a body in the absence of the rotation. §5. Equilibrium of a body with a fixed axis of the rotation in the planar case. Moment of the force. Test questions. Problems.
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