MOTION, FORCES, &c.
Body is the mass or quantity of matter in any material substance, and it is always proportional to its weight, or gravity, whatever its figure may be.
Density is the proportional weight, or quantity of matter in any body.
Velocity, or celerity, is an affection of motion by which a body passes over a certain space in a certain time.
Momentum, or quantity of motion, is the power, or force, in moving bodies.
Force is a power exerted on a body to move it, or to stop it. If the force act constantly, it is a permanent force, like pressure, or the force or gravity; but if it act instantaneously, or for an imperceptibly short time, it is called impulse, or percussion, like the smart blow of a hammer.
A motive, or moving force, is the power of an agent to produce motion.
Accelerative, or retardative force, is that which affects the velocity only, or it is that by which the velocity is accelerated, or retarded.
The change, or alteration of motion by any external force, is always proportional to that force, and in the direction of the right line in which it acts.