Newton's first law of motion is, in substance, that matter at rest will eternally remain at rest, and matter in motion will forever continue in motion, unless acted on by some external force.
A rider is carried over the head of a horse when the latter suddenly stops. This illustrates the inertia of movement. A stone at rest will always remain in that condition unless moved by some force. That shows the inertia of rest.
Momentum.—This is the term to designate the[p. 85] quantity of motion in a body. This quantity varies and is dependent on the mass, together with the velocity. A fly wheel is a good example. It continues to move after the impelling force ceases; and a metal wheel has greater momentum than a wooden wheel at the same speed, owing to its greater mass.
If, however, the wooden wheel is speeded up sufficiently it may have the same momentum as the metal one.
Weight.—All substances have what is called weight. This means that everything is attracted toward the earth by the force of gravity. Gravity, however, is different from weight. All substances attract each other; not only in the direction of the center of the earth, but laterally, as well.
Weight, therefore, has reference to the pull of an object toward the earth; and gravity to that influence which all matter has for each other independently of the direction.
Centripetal Force.—This attraction of the earth, which gives articles the property of weight, is termed centripetal force—that is, the drawing in of a body.
Centrifugal Force.—The direct opposite of centripetal, is centrifugal force, which tends to[p. 86] throw outwardly. Dirt flying from a rapidly moving wheel illustrates this.
Capillary Attraction.—There is a peculiar property in liquids, which deserves attention, and should be understood, and that is the name given to the tendency of liquids to rise in fine tubes.
It is stated that water will always find its level. While this is true, we have an instance where, owing to the presence of a solid, made in a peculiar form, causes the liquid, within, to rise up far beyond the level of the water.