Elementary Science 7

Source: Murche, Vincent. Science Readers, Book VII. London: MacMillan and Co., 1906.

Lesson 01. Matter in Motion

During the course of these lessons, we have been led, step by step, from a simple observation of the common things around us, to investigate some of the wonderful phenomena which are constantly going on before our eyes; and we have found that Nature works in her own appointed way, and by means of her own appointed agents, which we call the Natural Forces.

It will now be our province to enlarge the scope of those investigations, and to give a broader and deeper view of some of the wonderful workings of Nature. You, of course, remember that we have one general name – matter – for every kind of substance or material thing which exists in, on, and around our world; and one of our earliest observations led us to discover that matter exists in three distinct states. We have solid matter, liquid matter, and gaseous matter.

You know, too, that this is no mere accident, but one of Nature’s provisions. All matter is made up of molecules, and in a solid body, these molecules are held together by a mutually attractive force known as cohesion. But for this wonderful force, everything around us, even our own bodies, would fall away in an impalpable dust – nothing would hold together.

This force of cohesion is not so strong in liquids as in solids; hence the molecules of a liquid have more freedom of movement, and the liquid is said to flow. There is no cohesion between the molecules of a gas. They mutually repel each other.

We become familiar with the various forms of matter through one or more of the five gateways of knowledge, or in other words through the five senses. Some things appeal to us through the sense of sight, some through the sense of touch, some through the sense of taste, some through the sense of smell, and some through the sense of hearing.

By means of these channels of observation, we learn that all matter – solid, liquid, and gaseous – possesses certain properties. For example, all matter occupies space, has more or less weight, offers resistance, and transmits motion to other matter.

Let us for the present confine our attention to this last property, and it will perhaps help you to understand what that means, if we take a familiar illustration from the football field.

Picture the ball lying in the middle of the play at the commencement of the game. If it were not interfered with, it would remain where it lies, for like all other dead, helpless matter, it has no power of itself to move. It cannot move till some outside influence acts upon it.

It is the outside influence of the kick-off that sends it spinning towards the goal. It is the outside influence of the next kick, which changes the direction of its movement towards another part of the field. Here you see we have matter, in the form of the boy’s foot, transmitting motion to the dead, helpless matter of the ball.

What do you say? Why does the ball come to a standstill at last?

Yes, that is a very thoughtful question, and I will answer it.

The ball is brought to a standstill by the roughness of the ground, which offers resistance to it as it rolls along. This resistance, or friction as it is called, is another outside influence – another property of matter – but the effect of its action on the moving ball is to stop its movement and bring it to a standstill.

The same ball, rolling over a smooth even surface of ice, would encounter less resistance – less friction – and hence would travel farther, but even that slight resistance would be sufficient to bring it to rest at last.

All outside influence, whatever it may be, which causes one body to move and stops another already in motion, we shall henceforth call force. The particular force which sets the football in motion is the boy’s bodily strength, and this is known as muscular force.

Savage people of all ages have known very little of any other force but their own bodily strength, and that of the animals they have been able to subdue. But there are many wonderful forces at work around us, and civilized man has learned, and is still learning, how to adapt them to his service.

Just as the muscular force of the kick sets the ball in motion, or stops it when it is moving, so those other forces act upon all bodies – all matter – in nature, and produce similar results. Hence we speak of them as the Forces of Nature.

Let us take another illustration. You know that if you hold the ball out at arm’s length and let go, it falls to the ground. But why does it fall? Ah! I see you remember. It falls because the earth itself possesses a wonderful attractive force called gravity. All matter, if unsupported, falls to the ground because the earth’s force of gravity attracts it.

You see from this that gravity and cohesion are both attractive forces; but the force of gravity produces motion, while the attractive force of cohesion prevents it.

Just one more thought now before we leave this subject. Suppose I give you a heavy weight instead of the ball to hold at arm’s length. You could hold the ball for a considerable time, but as soon as you take the weight in your hand, you become conscious of a muscular strain.

The fact is the earth’s force of gravity is attracting the object, but your own muscular force is trying to resist the downward pull which gravity is exerting. If the object is very heavy, the force of gravity will quickly overcome your muscular force, the struggle will soon be over, and the thing will fall to the ground.

It is clear from this that gravity does not exert the same amount of force on all bodies. In other words, we speak of the force with which gravity attracts a body as the weight of that body; and we say the body is light or heavy in proportion to the force with which gravity attracts it.

Let me lastly point out that the earth exerts the same attractive force, whether the object is placed on a table, suspended from a cord, or supported in the hand. In the first case, it is the resistance offered by the table itself which overcomes the force of gravity and prevents the object from falling. In the second, it is the tension in the cord, and in the third, as we have seen, it is one’s muscular force.

Summary of the Lesson

1. Matter is the common name for every substance that exists.
2. All matter appeals to us through the senses.
3. Matter occupies space, has weight, offers resistance, and transmits motion.
4. Force is any influence which tends to move a body, to change the direction of its movement, or stop it when it is in motion.
5. The strength or force of men and animals is called muscular force.
6. Gravity and cohesion are both forces. Cohesion prevents motion; gravity produces it.
7. Gravity pulls some bodies with greater force than others.
8. The force with which gravity attracts a body is the weight of that body.

Lesson 02. The Forces of Nature

We have already had something to say about the two wonderful attractive forces, cohesion and gravity, and their individual action on matter of all kinds. Let us now turn our attention to some of the other Forces of Nature.

I suspend a steel knitting-needle in this paper stirrup and bring this steel bar near, and you see that the needle immediately moves towards the bar.

I need scarcely remind you that, as the needle is dead, helpless matter, it has no power in itself to move; neither will it make any attempt to move if I present the poker, a pointer, or any other object to it in place of the steel bar.

Yet the moment the bar is brought near, the needle moves towards it. Hence it is clear that this steel bar possesses some wonderful attractive influence over the needle, which does not belong to other bodies.

Now observe what happens if I remove the steel needle from the stirrup, and in place of it, put a piece of brass or copper wire, or this wooden penholder. There is no movement now when the steel bar is brought near, and that makes it equally clear that although this bar affects steel, it has no influence over brass, copper, or wood.

The influence which the steel bar exerts is known as magnetism, and it ranks with cohesion and gravity, as one of the Natural Forces; but unlike them, it does not act upon all matter. We shall have more to say about this later on.

Let us take another simple experiment. I rub this glass rod with a piece of warmed silk, and then bring the rubbed end near some scraps of paper on the table. The little bits of paper instantly fly up towards the rod.

Here again, we have a clear case of attraction. The rubbed glass attracts, or draws to itself, those light substances, by means of a wonderful force which it possesses.

Let us try it again with the other end of the rod, which has not been rubbed. There is no movement among the scraps of paper now, you see. Hence it is clear that the attractive force, which first caused the movement, was not originally in the glass – it must have been produced by rubbing the rod with silk.

This wonderful force – another of the Natural Forces – is known as electricity. It is closely allied to the other force – magnetism. We shall deal with both of them more fully by and by.

I think I have now made it clear to you that cohesion, gravity, magnetism, and electricity are all of them attractive forces.

Suppose we pass on from this to consider some of Nature’s other forces. I stand a flask of water over the Bunsen burner and watch it till it boils. What do I see? Particles of water move upwards in a constant stream to the surface under the influence of the heat.

Yes, I am sure you are all ready to explain the meaning of this. The effect of heat is to overcome the force of cohesion. The molecules of a body are driven apart under the influence of heat so that they occupy more space. As the heated molecules occupy more space than the rest, they are relatively lighter and are forced upwards by the buoyancy of the liquid.

Heat is another of the Natural Forces, but unlike those we have already considered, it exerts a repulsive, not an attractive influence.

We can see for ourselves this repulsive influence, as the water in the flask receives the heat from the burner. We can see the result of it too if we hold a bladder, partly filled with air, in front of the fire; for as the air inside becomes expanded with the heat, it spreads out and fills the bladder.

You no doubt remember our experiment with the brass ball and the ring. We could not, it is true, actually see the molecules of the brass ball move, but we know they must have moved because the ball, after it was heated, was too big to pass through the ring. Its molecules had been driven apart by the repulsive force of heat.

Heat, you remember, travels from one body to another in straight lines. These lines of heat we call rays, and we say that heat travels by radiation.

The sun, the source of all our heat, warms the earth by radiation; but we must also remember that the solar rays give light as well as heat, and this will naturally lead us to make some inquiry as to the nature of the solar rays themselves.

It is generally believed that the fiery orb of the sun is in a constant state of rapid vibration among its particles; and that the vast space between the sun and the earth is pervaded by an extremely thin, impalpable fluid, which, for want of a better name, has been called ether.

The phenomenon of light is then accounted for in this way. “The vibration in the substance of the sun itself sets up a series of waves in the surrounding medium – this all-pervading ether; and the ether-waves travel with a motion similar to that which

 would be produced, if one end of a long cord were tied to a post, and the other end were shaken or jerked up and down. “The ether-waves produced in this way travel with immense velocity, and when at last they strike on the retina of the eye, they give rise to the sensation of light. “The immense velocity of these ether-waves is shown by the fact that some of them make no less than 727 million millions of vibrations every second. “Light therefore, as an influence capable of producing motion, is rightly regarded as another of the Forces of Nature.”

But let us continue our investigations. I strike this tuning fork on the table, and while it is sounding, I will bring it lightly in contact with this sheet of paper. The fork is in a state of vibration; little taps can be distinctly heard as it vibrates against the paper. But if I press the paper against the fork, the quivering ceases, and so does the sound. If you strike the fork again, and while it is sounding bring it into contact with your teeth, the vibrations can be distinctly felt; but as soon as the vibrations cease, the sound ceases too.

This simple experiment will be quite sufficient to prove that sound is the result of vibration, and that without vibration there can be no sound. You all know the effect of throwing a stone into a pond of water. That will help you to form a good idea of the manner in which these sound vibrations are communicated.

The vibrations set up in the sounding body are communicated to the air around by a series of waves, which spread out in widening circles, in some such way as the circles spread in the water. When these air-waves reach the ear, and strike on the nerve of hearing, they give rise to the sensation of sound.

Summary

1. Cohesion, gravity, magnetism, and electricity are attractive forces.
2. Heat expands bodies by driving its molecules farther apart.
3. Heat is a cause of motion; it is, therefore, one of Nature’s forces.
4. Light is conveyed through space by the rapid vibration of the ether-waves.
5. It is a cause of motion; and is therefore another of Nature’s forces.
6. Sound is the result of vibration.
7. It is conveyed by a series of waves, which spread out in circles through the air.

Lesson 03: The Builder’s Plummet

We know that matter of all kinds possesses weight, although some bodies have more weight than others; and we have found that this property of weight is due to the force of gravity, which attracts all matter towards the earth.

The weight of a body causes a downward pressure, which is in proportion to the force with which gravity attracts it, and if we represent the direction of this downward pressure by a line, we say that the line is vertical or upright.

No doubt you have all seen a plumb-line or plummet, and you know that the builder uses the instrument to test the uprightness of his work.

He can see at a glance whether the wall he is building is upright or not, because the cord of the plummet always hangs in a vertical or upright line. And yet it is a very simple contrivance, for it consists merely of a cord with a weight attached to one end, and every similarly-weighted cord hangs in the same direction.

Tie a stone to the end of a string, and set it swinging from side to side. Then if you let it come to rest, you will find that it always returns to the same position, with the string hanging in a vertical line.

Now hold the cord so as to allow the stone to dip into the water in this bowl, and you see at once that the surface of the water is at right angles to the vertical line of the hanging cord.

It would be exactly the same if we allowed the stone to dip down into a pond, a lake, or any other great body of water on any part of the earth’s surface. The downward pressure of the stone, owing to the attractive force of gravity, would cause the cord to hang in a vertical line, and at right angles to the surface of the water.

We say that the surface of the water forms a horizontal plane, and this is always at right angles to the vertical cord.

We often use these two terms – horizontal and vertical. Let us see what they really mean.

With the help of the chalk, and a piece of string to act as a radius, draw on the blackboard or on a wall a very large circle. Then fix upon some point outside the circle, and from it rule a straight line to the centre. Then from the spot where the line cuts the circle measure off a small piece of the circumference, say half an inch on either side.

That done you will no doubt be very much surprised, if you place the edge of a straight ruler close up against this small section of the circle, for you will find that this little piece of the circle is as straight as the ruler itself.

Strange as it may seem, it is still perfectly true that any small piece of the line, in any part of the circumference, appears to be quite straight.

The fact is, the piece is so small compared with the size of the circle itself that we do not notice any roundness or curve in it.

Picture to yourselves, as a further illustration, a great ball, say a yard across, and suppose we could cut away a piece of its surface about the size of a sixpence, just as we remove a piece of the shell from an egg.

That little piece of the round ball would lie flat on the table, for it would itself be quite flat.

Now let us apply this to the great ball on which we live. Our earth is 25,000 miles round, and at the best we can see only a very small part of its surface at one time. The part of it which we see is like the little piece of the ball. It appears quite flat, and this is particularly noticeable out on the open sea, where there is nothing to obstruct the view, for there, on every side, stretches the same level, horizontal surface of water.

It is this apparent flatness of the earth’s surface, which makes it so difficult for young people to believe that they live on an immense ball.

Let us turn once more to our drawing on the board. That little section of the circle, which lies straight and even with the ruler, represents the level or horizontal surface of the earth or water at any particular spot, and you know that the vertical line at right angles to it passes through the centre of the circle.

Now imagine yourself anywhere you please on the earth’s surface with the plummet in your hand. The plumb-line, of course, would hang downwards, and in a vertical direction, and it is perfectly clear that, if the line of that direction were continued, it must pass through the centre of the earth.

I think you will now clearly understand that every vertical line at every spot on the earth’s surface must, if produced, pass through its centre; and that bodies hang and fall in a vertical line, because the force of gravity attracts them to the earth’s centre.

This makes it quite clear that the centre of the earth is the centre of the earth’s force of gravity.

Summary of the Lesson:

1. Suspended bodies hang in a vertical line.
2. A horizontal plane is at right angles to the vertical line of a plummet.
3. The vertical line of the plummet, if produced, would pass through the centre of the earth.
4. The centre of the earth is the centre of its force of gravity.

Lesson 04. Gravitation

In our last lesson, the builder’s plummet made it clear that the centre of the earth is the centre of the earth’s force of gravity.

We are now in a position to carry our investigations a step farther, and with that aim in view, let us refer once more to the ball or stone suspended at the end of the string.

When you hold the cord in your hand with the ball at the opposite end, it hangs in a vertical line; and you observe that if I cut the cord through with the scissors, the ball falls to the floor in the same vertical line.

It falls in that direction because it is attracted by the force of gravity which acts from the centre of the earth. You have already seen that every vertical line, at every spot on the earth’s surface, must, if produced, proceed direct to its centre.

Of course, you have heard the story of Sir Isaac Newton and the falling apple. He had, no doubt, seen many an apple fall from trees before, but on that occasion, it set him thinking.

He began by asking himself the question, “Why did the apple fall to the ground, and not away from it?” This question, simple as it seems, led him on not only to investigate the earth’s force of gravity but to discover, step by step, the mutual relation which exists between the earth, the sun, the moon, and the other heavenly bodies.

After many years of patient experiment and research, he came to the conclusion that there is a universal force in Nature, by virtue of which every material body attracts every other material body, and this universal force he called gravitation.

He further discovered that the attraction between any two bodies acts in a line joining their centres, and that the attraction exerted by each body is directly proportioned to its mass, or in other words, the greater the mass of matter, the greater the attractive force which it possesses.

Now let us apply this to the suspended ball. You already know that the earth attracts the ball; but it is equally important to remember that the ball in its turn attracts the earth, their mutually attractive forces acting in a line joining their centres.

The tendency of this mutual attraction is to cause the ball and the earth to move towards each other along this line.

But the mass of matter in the earth is immeasurably greater than the mass of matter in the ball, and therefore the force with which the earth attracts the ball is immeasurably greater than that with which the ball attracts the earth.

Hence, if we say that the ball moves as much farther than the earth, as the earth’s mass is greater than the mass of the ball, we practically mean that the earth remains stationary, and only the ball moves when the cord is cut.

Sir Isaac Newton, as I have already observed, did not stop here. He discovered that this force of gravitation affects every material body throughout the universe; and we are thus enabled to account for the movements of the earth and the other planets in their orbits round the sun.

The sun attracts the earth, and the earth in its turn attracts the sun, their mutually attractive forces acting in the line which joins their centres.

But the mass of matter in the sun is vastly greater than the mass of matter in the earth. Hence the attraction which the sun exerts on the earth is vastly greater than that which the earth exerts on the sun.

For practical purposes, therefore, we may regard the sun as a stationary body, with the earth and the planets moving round it, and the mutual attraction of gravitation between it and them keeps them in their regular paths or orbits.

Newton was able at last to embody the results of his investigations in three statements, and these he called the Laws of Gravitation. They are:

1. Every material body in Nature attracts every other material body at all distances.
2. For the same distance, the attractions between bodies are directly proportioned to their masses.
3. The intensity of this attraction varies inversely with the square of the distance.

The first and second of these laws have already been dealt with. Let us see what we can make of the third.

Of course, you know that the square of any number is the product obtained by multiplying that number by itself. Thus 4 = 2 X 2; 9 = 3 x 3; 100 = 10 X 10, and therefore 4, 9, and 100 are respectively the squares of 2, 3, and 10.

You know, too, that when we turn a thing upside down, we are said to invert it, as you do in Arithmetic when you invert a fraction. Thus ¾ when it is inverted, becomes 4/3, and we say 4/3 is the inverse of ¾. Similarly, ¼, 1/9, and 1/100 are respectively the inverse of the squares of 2, 3, and 10.

Now let us see what Newton’s third law really means.

It means that if the distance between two bodies is doubled, the attraction of gravitation between them is only ¼ of what it was; if the distance is tripled, the attractive force is diminished to 1/9 of its original intensity; and if it is increased 10 times, the attraction is only 1/100 of the original.

Summary

1. Gravitation is the general name for the force by which every material body in the universe attracts every other material body.
2. The laws of gravitation are:
   (a) Every material body attracts every other material body at all distances.
   (b) For the same distance, the attractions between bodies are directly proportioned to their masses.
   (c) The intensity of this attraction varies inversely with the square of the distance.