CONVERSATION III.

ON THE LAWS OF MOTION.

On Motion. Of the Inertia of Bodies.-Of Force to Produce Motion.-Direction of Motion.-Velocity, Absolute and Relative.-Uniform Motion.-Retarded Motion-Accelerated Motion.-Velocity of Falling Bodies. Momentum.-Action and Re-action Equal. -Elasticity of Bodies.-Porosity of Bodies.-Reflected Motion.-Angles of Incidence and Reflection.

MRS. B.

THE Science of mechanics is founded on the laws of motion; it will, therefore, be necessary to make you acquainted with these laws before we examine the mechanical powers. Tell me, Caroline, what do you understand by the word motion ?

Caroline. I think I understand it perfectly, though I am at a loss to describe it. Motion is the act of moving about, going from one place to another, it is the contrary of remaining at rest.

Mrs. B. Very well. Motion then consists in a change of place; a body is in motion whenever it is changing its situation with regard to a fixed point.

Now since we have observed that one of the general properties of bodies is Inertia, that is, an entire passiveness either with regard to motion or rest, it follows that a body cannot move without being put into motion; the power which puts a body into motion is called force; thus the stroke of the hammer is the force which drives the nail; the pulling of the horse that which draws the carriage, &c. Force then is the cause which produces motion.

Emily. And may we not say that gravity is the force which occasions the fall of bodies?

Mrs. B. Undoubtedly. I had given you the most familiar illustrations in order to render the explanation clear; but since you seek for more scientific examples, you may say that cohesion is the force which binds the particles of bodies together, and heat that which drives them asunder.

The motion of a body acted upon by a single force is always in a straight line, in the direction in which it received the impulse.

Caroline. That is very natural; for as the body is inert, and can move only because it is impelled, it will move only in the direction in which it is impelled. The degree of quickness with which it moves, must, I suppose, also depend upon the degree of force with which it is impelled.

Mrs. B. Yes; the rate at which a body moves, or the shortness of the time which it takes to move from one place to another, is called its velocity; and it is

one of the laws of motion that the velocity of the moving body is proportional to the force by which it is put in motion. We must distinguish between absolute and relative velocity.

The velocity of a body is called absolute, if we consider the motion of the body in space, without any reference to that of other bodies. When for instance a horse goes fifty miles in ten hours, his velocity is five miles an hour.

The velocity of a body is termed relative, when compared with that of another body which is itself in motion. For instance, if one man walks at the rate of a mile an hour, and another at the rate of two miles an hour, the relative velocity of the latter is double that of the former; but the absolute velocity of the one is one mile, and that of the other two miles an hour.

Emily. Let me see if I understand it. The relative velocity of a body is the degree of rapidity of its motion compared with that of another body; thus if one ship sail three times as far as another ship in the same space of time, the velocity of the former is equal to three times that of the latter.

Mrs. B. The general rule may be expressed thus: the velocity of a body is measured by the space over which it moves, divided by the time which it employs in that motion: thus if you travel one hundred miles in twenty hours, what is your velocity in each hour?

Emily. I must divide the space, which is one hundred miles, by the time, which is twenty hours, and the answer will be five miles an hour. Then, Mrs. B., may we not reverse this rule and say, that the time is equal to the space divided by the velocity; since the space

one hundred miles, divided by the velocity five miles, gives twenty hours for the time?

Mrs. B. Certainly; and we may say also that space is equal to the velocity multiplied by the time. Can you tell me, Caroline, how many miles you will have travelled, if your velocity is three miles an hour and you travel six hours ?

Caroline. Eighteen miles; for the product of S multiplied by 6, is 18.

Mrs. B. I suppose that you understand what is meant by the terms uniform, accelerated and retarded

motion.

Emily. I conceive uniform motion to be that of a body whose motion is regular, and at an equal rate throughout; for instance, a horse that goes an equal number of miles every hour. But the hand of a watch is a much better example, as its motion is so regular as to indicate the time.

Mrs. B. You have a right idea of uniform motion; but it would be more correctly expressed by saying, that the motion of a body is uniform when it passes over equal spaces in equal times. Uniform motion is produced by a force having acted on a body once, and having ceased to act; as for instance, the stroke of a bat on a cricket ball.

Caroline. But the motion of a cricket ball is not uniform; its velocity gradually diminishes till it falls to the ground.

Mrs. B. Recollect that the cricket ball is inert, and has no more power to stop than to put itself in motion; if it falls, therefore, it must be stopped by some force superior to that by which it was projected, and which destroys its motion.

Caroline. And it is no doubt the force of gravity which counteracts and destroys that of projection; but if there were no such power as gravity, would the cricket ball never stop?

Mrs. B. If neither gravity nor any other force, such as the resistance of the air, opposed its motion, the cricket ball, or even a stone thrown by the hand, would proceed onwards in a right line, and with an uniform velocity for ever.

Caroline. You astonish me! I thought that it was impossible to produce perpetual motion ?

Mrs. B. Perpetual motion cannot be produced by art, because gravity ultimately destroys all motion that human powers can produce.

Emily. But independently of the force of gravity, I cannot conceive that the little motion I am capable of giving to a stone would put it in motion for ever.

Mrs. B. The quantity of motion you communicate to the stone would not influence its duration; if you threw it with little force it would move slowly, for its velocity, you must remember, will be proportional to the force with which it is projected; but if there is nothing to obstruct its passage, it will continue to move with the same velocity, and in the same direction as when you first projected it.

Caroline. This appears to me quite incomprehensible; we do not meet with a single instance of it in nature.

Mrs. B. I beg your pardon. When you come to study the motion of the celestial bodies, you will find that nature abounds with examples of perpetual motion; and that it conduces as much to the harmony of the