Mechanics is concerned with the behaviour of matter under the influence of forces. It may be divided into statics, which is concerned with matter at rest, and dynamics, which deals with matter in motion. Dynamics may in turn be divided into kinematics, the study of motion regardless of its cause, and kinetics, the study of motion and force. Mechanics may also be divided into solid and fluid mechanics.
In statics, the forces on an object are balanced and the object is in equilibrium (stationary: the forces on an object moving at constant velocity are balanced as well, but that situation is not covered by statics). Static equilibrium may be stable, where an object displaced from its position returns to that position; unstable, where the object accelerates away from its previous position; or neutral, where it rests in a new position.
The mass (m) of a body is a measure of the quantity of matter it contains; the SI unit of mass is the kilogram (kg). Mass is not the same as weight, which for our purposes is the force of attraction of the Earth on that body.
A rigid body is one that does not deform. In this chapter, all bodies are considered rigid: deformation is covered in the chapter on Stress Analysis. In some cases, it may be plausible to treat an object as a particle, a body of negligible size.
This chapter shall cover the behaviour of matter at speeds significantly below that of light. Relativistic mechanics deals with matter at speeds approaching that of light, while quantum mechanics is concerned with matter at the level of atoms and molecules.
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