ATTRACTION. 4. ATTRACTION denotes the property which bodies have to approach to each other. There are five kinds of Attraction: 1, the attraction of cohesion; 2, of gravitation; 3, of electricity; 4, of magnetism; 5, of chemical attraction. Cohesion is exerted only at very small distances, its strength varies in different kinds of matter, and is supposed to be the cause of the relative degrees of hardness of different bodies. Capilliary Attraction is only a particular modification, or branch of the attraction of cohesion. GRAVITATION decreases from the surface of the earth upwards, as the square of the distance increases; but from the surface of the earth downwards, it decreases only in a direct ratio to the distance from the centre. REPULSION. 1. REPULSION is that property in bodies, whereby, if they are placed just beyond the sphere of each other's attraction of cohesion, they mutually fly from each other. 2. Oil refuses to mix with water, from the repulsion between the particles of the two substances; and from the same cause, a needle gently laid upon water will swim. MOTION. Absolute Motion, is the actual motion that bodies have, considered independently of each other, and only with regard to the parts of space. Relative motion, is the degree and direction of the motion of one body, when compared with that of another. Accelerated motion, is understood, when its velocity continually increases. Retarded motion, when the velocity continually decreases, and the motion is said to be uniformly retarded. when it decreases equally in equal times. Uniform motion is estimated by the time employed in moving over a certain space, or in other words, by the space moved over in a certain time. CASE I. To ascertain the velocity. Divide the space run over by the time. CASE II. To ascertain the distance. Multiply the velocity by the time. In accelerated motion the space run over (or distance) is as the square of the time. 1. A body acted upon only by one force, will always' move in a straight line. 2. Bodies acted upon by two single impulses, whether equal or unequal, will also describe a right line. The Momentum of a body is the force with which it moves, and is in proportion to the weight or quantity of matter, multiplied into its velocity. The action of bodies on each other are always equal, and exert in opposite directions; so that any body acting upon another, loses as much force as it communicates. CENTRAL FORCES. The central forces are its centrifugal and centripetal forces. The centrifugal force is the tendency which bodies that revolve round a centre, have to fly from it in a tangent to the curve they move in, as a stone from a sling. 2. The centripetal force is that which prevents a body from flying off by impelling it towards the centre, as the attraction of gravitation. CENTRE OF GRAVITY. 1. The centre of gravity is that point in a body about which all its parts exactly balance each other in every direction. 1. A vertical line passing through the centre of gravity of a body, is called the line of direction. 2. When the line of direction falls within the base of a body, that body cannot descend; but if it falls without the base, the body will fall. MECHANICAL POWERS. That body which communicates motion to another is called the power. The body which receives motion from another, is called the weight. The mechanical powers are five the lever, the wheel or acle, the pully, the screw and the wedge, to these, may be added the inclined plane. OF LEVERS. There are 3 orders or varieties of levers, wherein the weights, props or moving powers, may be differently applied to the vectis or inflexible bar, in order to effect mechanical operations in a convenient manner. A lever is said to be of the first order when the prop is between the weight and the power; of the second order, when the weight is between the prop and the power; of the third order, when the power is between the prop and the weight. A power and weight acting upon the arms of a lever will balance each other, when the distance of the point at which power is applied to the lever from the prop, is to the distance of the point at which weight is applied, as the weight is to the power. To find what weight may be raised by a given power. RULE.- As the distance between the body to be raised or balanced, and the fulcrum or prop, is to the distance between the prop and the point, where the power is applied, so is the power to the weight which it will balance. 1. If a lever be 100 inches long, wolat weight lying 71 inches from the end, resting on a pavement, may be moved with the force of 168 lbs. lifting at the other end of the lever? Ans. 2072 lbs. 2. A water wheel turns a crank working three pump rods, fixed 6 feet from the joint or pin which their several levers, each 9 feet in length are fastened on account of the intended motion at one end, the suckers of the pumps being put in operation by the other, proves them to be levers of the third order. Now if the crank in this case plays 9 inches round its centre, what is the length of the stroke in each of the barrels? Ans. 27 inches. 3. With what force of water ought that water wheel be driven, (circumstanced as in the last question) which raises 3 cubic feet of water at every revolution of the wheel, each experimentally weighing 62 lbs. Avoirdupois, the friction of the wheel rejected? Ans. 281ị lbs. 4. At what distance from a weight of 1530 lbs. must a prop be placed, so that a power of 170 pounds, applied 9 feet from the prop, may balance it? Ans. 1 foot. THE AXLE OR WHEEL. The wheel or axle is a wheel turning round together with its axis the power is applied to the circumference of the wheel, and the weight to that of the axis, by means of cords, an equilibrium is produced in the wheel and axis, when the wheel is to the power, as the diameter of the wheel to the diameter of its axis. RULE.- As the diameter of the wheel is to the diameter of the axle, so is the weight to be raised by the axle, to the power that must be applied to the wheel. CASE I. 1. If the diameter of the axle be 6 inches, and the diameter of the wheel 4 feet, what power must be applied to the wheel to raise 960 lbs. at the axle? Ans. 120 lbs. CASE II. 2. If the diameter of the axle be 6 inches, and the diameter of the wheel 5 feet, what power must be applied to the axle to raise 300 lbs. at the wheel? Ans. 2400lbs. CASE IIL 3. If the diameter of the axle be 8 inches, and 300 lbs. applied to the wheel to raise 2400 lbs. at the axle, what is the diameter of the wheel? Ans. 64 inches. CASE IV. 4. If the diameter of the wheel be 64 inches and 300 lbs. applied to the wheel to raise 2400 lbs. at the axle, what is the diameter of the axis? Ans. 8 inches. PULLEY. The pulley is a small wheel moveable about its axis by means of a cord which passes over it. When the axis of the pully is fixed, the pully only changes the direction of the power; if moveable pulleys are used an equilibrium is produced when the power is to the weight, as one to the number of ropes applied to them, if each moveable pully has its own rope, each pully will be double the power. To find the weight that may be raised by a given power. RULE.—Multiply the power by twice the number of moveable pulleys, and the product is the weight. CASE I. 1. What power must be applied to a rope that passes over one moveable pully, to balance a weight of 400 lbs.? Ans. 200 lbs. CASE II. 2. What weight will be balanced by a power of 20 lbs. attached to a cord that passes over three moveable pulleys? 23* Ans. 120 lbs. |