The proper significance of the principle of virtual work, and of its converse, will appear more clearly when we come to **kinetics** (~ 16); for the present it may be regarded merely as a compact and (for many purposes) highly convenient summary of the laws of equilibrium.

Now the **kinetics** of a medium in which the parts can have finite relative motions will lead to equations which are not linear - as, for example, those of hydrodynamics - and the phenomena will be far more complexly involved.

The subject is usually expounded under the two divisions of statics and **kinetics**, the former dealing with the conditions of rest or equilibrium and the latter with the phenomena of motion as affected by force.

**Kinetics** of a system of discrete particles.

**Kinetics** of a rigid body.

The question as to stability of equilibrium belongs essentially to **kinetics**; but we may state by anticipation that in cases where gravity is the only force which does work, the equilibrium of a body or system of bodies is stable only if the depth of the centre of gravity be a maximum.

**Kinetics** of a System of Discrete Particles.The momenta of the several particles constitute a system of localized vectors which, for purposes of resolving and taking moments, may be reduced like a system of forces in statics (~ 8).

**Kinetics** of a Rigid Body.

Moving A xes of ReferenceFor the more general treatment of the **kinetics** of a rigid body it is usually convenient to adopt a system of moving axes.

The most important conception in **kinetics** is that of inertia.