Upon what does the attraction of gravitation in a body depend?
It depends upon the size and weight of that body.
Do larger bodies attract with more force than smaller ones?
Yes; larger bodies do attract more than smaller ones.
Do heavy bodies attract with more force than light ones?
Yes; heavy bodies do attract more than light ones, although they may be of the same size.
Where are all substances drawn by the attraction of gravitation?
They are all drawn towards the centre of the earth.
Why are substances drawn towards the centre of the earth?
Because the earth is larger than any substance on or near its surface.
In what direction do all objects fall through the air?
They all fall towards the centre of the earth.
Which way then do we call “down”?
Downis always towards the centre of the earth.
Which way do we call “up”?
Upis always away from the centre of the earth.
Where must all plumb lines or perpendicular lines point?
They must all point with one end towards the centre of the earth.
When the earth turns over, can any substance fall away from its surface?
No; all things are drawn to the earth with so much force that they cannot fall away.
Upon what does the weight of a body depend?
The weight of a body depends upon the force with which it is drawn towards the earth.
What kind of substances are attracted with the most force?
Those that are the most dense, like the metals; hence, they are the heaviest.
What causes a liquid to form into a drop at the mouth of a bottle?
The attraction of cohesion causes the drop to be formed.
What causes the drop to fall to the earth?
The attraction of gravitation causes the drop to fall to the earth.
When water is poured from a tumbler, why does it run down the side of the tumbler?
Because the attraction of cohesion draws the water to the side of the tumbler, as is shown inFig. 27.
Fig. 27.
Fig. 27.
Why does a spout prevent the water from running down the side of the vessel?
Because the spout is curved so that its lowest point is where the water falls from it; hence, gravitation causes the water to fall directly to the earth, as is shown inFig. 28.
Fig. 28.
Fig. 28.
Does the air obstruct bodies when falling through it?
Yes; air obstructs all bodies when falling through it; but it obstructs light bodies more than heavy ones.
Do all bodies fall with equal rapidity in a vacuum?
Yes; all bodies fall with equal rapidity in a vacuum.
What bodies fall through the air most rapidly?
Those that are the heaviest or most dense; because they are the smallest in bulk.
Do heavy bodies increase in velocity as they descend?
They do increase rapidly in velocity as they descend.
How far does a heavy body like lead fall in one second?
It falls sixteen feet in one second.
A falling body increases thirty-two feet in velocity for every second of time it is in falling. During the first second it falls sixteen feet. During the second second, it falls sixteen feet plus thirty-two feet, or forty-eight feet. During the third second, it falls forty-eight feet plus thirty-two feet, or eighty feet, etc.
A rising body decreases thirty-two feet in velocity for every second of time it is in rising. If it be three seconds in rising, its velocity will be eighty feet the first second, forty-eight feet the second second, and sixteen feet the third second, being just the reverse of the same body when falling, as is shown inFig. 29.
Fig. 29.
Fig. 29.
How may the distance a body falls in a given time be found?
It may be found by multiplying the number of seconds it is in falling, by the same number, and this product by sixteen feet. Thus, when it is three seconds in falling, multiply three by three, and this product by sixteen feet, which gives one hundred and forty-four feet, the distance it falls in three seconds.
What familiar example have we of the force of falling bodies?
We have an example in the large hail-stones which fall to the earth in summer-time, breaking the glass in our windows and doing much injury to the crops.
What is the centre of gravity in a body?
Thecentre of gravityin a body, is the point on which the body may be balanced, as is shown inFig. 30.
Fig. 30.
Fig. 30.
Where is the centre of gravity in a circle?
It is the centre of the circle.
Where is the centre of gravity in a ball?
It is the centre of the ball.
What influence does the earth have on the centre of gravity?
The earth draws the centre of gravity in a body as near to itself as possible.
Why will an egg, when placed on either end, fall over on its side?
Because the centre of gravity is nearer the earth when the egg is placed upon its side than when upon either end, as is shown inFig. 31andFig. 32.
Fig. 31.
Fig. 31.
Fig. 32.
Fig. 32.
Why will a ball remain in any position when placed on the earth?
Because its centre of gravity is always the same distance from the earth, whatever may be its position.
When will a body change its position if left unsupported?
A body will change its position whenever by so doing its centre of gravity will be lowered.
When may a body be readily moved about?
When its centre of gravity always remains at the same distance from the earth while being moved.
Why is a round body, like a ball, easily rolled about the floor?
Because its centre of gravity remains at the same distance from the floor.
When is the position of a body difficult to change?
Whenever the centre of gravity in it has to be lifted up in order to make the change, as is shown inFig. 35.
Fig. 33.
Fig. 33.
Fig. 34.
Fig. 34.
Fig. 35.
Fig. 35.
The centre of gravity, inFig. 33, may be lowered; hence, it will change its position if unsupported. The centre of gravity, inFig. 34, can neither be lowered nor lifted up; hence, it may be readily rolled about. The centre of gravity, inFig. 35, must be lifted up; hence, it remains firm in its place.
When does an object stand firm?
It stands firm when it has a broad base, and its centre of gravity is near the earth.
What effect is produced by elevating the centre of gravity?
The higher the centre of gravity in any body is raised above the earth, the greater is its tendency to fall.
Why is a load of hay more easily overturned than a load of iron, when upon the same wagon?
Because the centre of gravity in the load of hay is much higher than it is in the load of iron.
Why do passengers on top of a stage cause it to overturn more easily?
Because their weight on the top raises the centre of gravity in the stage, and it is more readily overturned.
Why is a person more likely to fall when standing up than when sitting down?
Because the centre of gravity is higher in a person when standing than when sitting.
Why is a boy more likely to fall when raised on stilts?
Because the centre of gravity in the boy is farther from the earth when he is raised on stilts.
When will a load of hay be overturned?
It will be overturned whenever the line from the centre of gravity to the earth falls outside of the wheels, as is shown inFig. 36.
Fig. 36.
Fig. 36.
Why should lamps have large bottoms?
Lamps should have large bottoms so that the line of gravity may not fall outside of the bottom.
Where must the line of gravity in a body always fall?
It must always fall within the base of that body, or the body will be overturned.
How must a leaning tower be built so as not to fall?
It must be built so that the line of gravity will fall within its base.
Why do we lean back when carrying a load in front of us?
We lean back so that the line of gravity may be kept directly over our feet.
Why does a person standing up in a wagon fall when the wagon is suddenly moved?
Because the feet, being in contact with the wagon, move with it before the motion extends to the rest of the body, thus throwing the line of gravity behind the feet; hence, the person must fall, unless supported.
Why does a person standing up in a wagon fall when the wagon is suddenly stopped?
Because the feet, being in contact with the wagon, are suddenly stopped, while the body still moves forward, thus throwing the line of gravity ahead of the feet; hence, the person must fall, unless supported.
Why do animals withfourfeet stand firmer than those with onlytwofeet?
Because the four feet afford a larger base to stand upon than the two feet do, and, therefore, support the centre of gravity better.
Why can a horse or a cow remain standing for many hours without seeming to be weary?
Because their four feet afford a good support to the centre of gravity.
Why does a person who is standing soon become weary?
Because it requires a constant effort to keep the line of gravity over the feet, and this soon produces weariness.
Why can a person sit longer than he can stand?
Because the centre of gravity is lower, when he is sitting, than when he is standing; hence, it requires less strength to support the body in a sitting position.
Why can a person lie down longer than he can sit or stand?
Because the centre of gravity is nearest the earth when he is lying down, and it requires no effort to remain in that position.
How do we measure time?
We measure time by means of apendulum, such as is used in a common clock.
How does the pendulum move?
The pendulum moves regularly to and fro, from the line of gravity.
Does the pendulum vary any in its vibrations, or the time of moving to and fro?
The pendulum does not vary any in its vibrations.
Fig. 37.
Fig. 37.
Why does the pendulum keep moving?
When the pendulum is at A, inFig. 37, gravity causes it to descend to C, and the motion it gains while falling to C carries it up to B; gravity then causes it to descend to C, and themotion it gains while falling to C carries it back to A again.
What helps to keep the pendulum in motion?
The works inside of the clock help to keep the pendulum in motion.
How long must the pendulum be, so as to tick once in every second?
The pendulum must be thirty-nine inches long.
How does it tick when the pendulum isshortened?
It ticks morerapidly.
How does it tick when the pendulum islengthened?
It ticks moreslowly.
How then doescoldaffect a clock?
Cold makes the clock go faster by shortening the pendulum.
How doesheataffect a clock?
Heat makes the clock go more slowly by lengthening the pendulum.
How may the pendulum always be kept of the same length?
By having the weight at the bottom of the pendulum, so that it may be raised or lowered by means of a screw.
Upon what does the weight of a body, when in the air, depend?
The weight of a body depends upon the force with which it is drawn towards the earth.
What is this attractive force called?
It is called gravitation or gravity.
What is gravity?
Gravityis the weight of any body.
What is specific gravity?
Specific Gravityis the weight of any body, compared with an equal bulk of water.
Do substances weigh as much in water as they do in air?
No; they do not weigh so much.
When we drop lead in a vessel full of water, what becomes of the water?
The water is forced over the sides of the vessel.
How much water is thus forced over the sides of the vessel?
A quantity equal in bulk to the lead dropped into the vessel.
When the water thus displaced weighs just one pound, how much less will the lead weigh while in the water than it did in the air?
It will weigh one pound less.
Then how much less is the weight of a body in water than it is in air?
Its weight in the water is as much less as the weight of the water which it displaces.
When a body has greater specific gravity than water, what will it do?
It will sink in the water.
When it has less specific gravity than water, what will it do?
It will float on the surface of the water.
Will all parts of the body be above the surface of the water?
No; a part of it will sink into the water.
How much of it will sink into the water?
It will sink so as to displace water enough to balance its own weight.
Why does a boat float on the water?
Because it weighs less than a bulk of water of the same size as the boat.
When a cargo is placed on the boat, why does it still float?
Because it still weighs less than a bulk of water of the same size as the boat.
How do we make use of this principle?
We make use of it in the construction of vessels, so as to carry on trade with other parts of the world.
Why can ducks and geese float on the water?
Because their light, downy feathers make their specific gravity less than that of the water; hence, they must float on its surface.
What may be said of the specific gravity of the human body?
When the lungs are filled with air, the specific gravity of the human body is rather less than that of water.
Can a person float in the water?
Yes; if a person, who cannot swim, will throw his head back so that his mouth and nose only remain above the water, he may float in this position for some time.
What is the specific gravity of fish?
The specific gravity of fish is so nearly equal to that of water, that they can move about anywhere beneath its surface.
How do fish rise and sink in the water?
Fish rise and sink in the water by means of an air-bladder in their bodies.
How does the air-bladder help the fish to rise or sink?
When the bladder is filled with air, the specific gravity of the fish becomes lighter, and it rises towards the surface; and when the air is forced out, the specific gravity becomes heavier, and the fish sinks towards the bottom.
Why does cream rise to the surface of milk?
Because the specific gravity of the cream is less than that of the milk.
Why does oil float on water?
Because the specific gravity of the oil is less than that of the water.
Why can a fat person float more easily in the water than a lean person?
Because the fat reduces the specific gravity of the body, so that it floats more easily.
Why can a person float more easily in salt water than in fresh water?
Because salt increases the weight of the water, and, therefore, it buoys up with more force.
What familiar example have we of this?
An egg will sink when placed in fresh water; but if salt be dissolved in the water, the egg will float near the surface.
Decoration.
Decoration.
Illustrated Capital W
When is a body said to be in motion?
A body is inmotionwhen it is moving from one place to another.
When is a body said to be at rest?
A body is at rest when it is not moving from one place to another.
What is necessary to give motion to a body?
Force of some kind is necessary to give motion to a body.
How long will the motion given to a body continue?
The motion will continue until it is checked.
What is necessary to check the motion of a body?
Force of some kind is necessary to check its motion.
What force is always acting so as to bring bodies to a state of rest?
The force of gravity, which is constantly drawing all bodies to the earth.
What else may check the motion of a body?
The air, the water, and solids of all kinds may check the motion of a body.
In what direction will a body move when its course is not obstructed?
It will move in a straight line.
Why does a ball struck by a bat make a curve to the earth?
Because the force of gravity gradually overcomes the force given by the bat, so that the ball moves in a curve.
Why does a bullet make a curve to the earth?
Because the force of gravity gradually overcomes the force given by the powder, so that the bullet moves in a curve to the earth.
When a person aims directly at a mark some distance from him, where will the bullet strike?
The bullet will strike below the mark.
Why will the bullet strike below the mark?
Because the force of gravity draws it towards the earth, while it is going from the gun to the mark.
When a ball attached to a string is whirled around, as inFig. 38, what is the tendency of the ball?
The tendency of the ball is to fly off in a straight line.
Fig. 38.
Fig. 38.
What is the force tending to make the ball fly off called?
It is calledcentrifugalforce.
What keeps the ball from flying off?
The string keeps the ball from flying off.
What is the force exerted by the string called?
It is calledcentripetalforce.
What then is centrifugal force?
It is the force tending to fly off from the centre.
What is centripetal force?
It is the force tending to draw towards the centre.
Why does mud fly off from the wheel of a wagon when in motion?
Because the motion of the wheel gives centrifugal force to the mud, and causes it to fly off.
What does the rim of a wheel when in motion, tend to do?
The rim tends to fly off from the centre of the wheel.
How is it prevented from flying off?
It is prevented by making the wheel very strong, so that all parts of it are held firmly together.
What does this centrifugal force sometimes cause?
The centrifugal force is so great that it sometimes causes grind-stones and mill-stones, when revolving rapidly, to burst into pieces.
What becomes of these stones when they burst?
When they burst, their pieces fly off in different directions, doing more or less injury to whatever obstructs their course.
When a horse turns a corner, why does the rider lean in the direction in which the horse is turning?
He leans so as to overcome the centrifugal force, and is thus prevented from falling off the horse.
How are cars prevented from running off the track, while turning a curve?
The outer rail is made higher than the inner rail, so as to overcome the tendency of the cars to run off the track.
What grand example have we of the action of these two forces?
The earth and other planets that move around the sun, are kept in their places by the action of these two forces.
When a ball strikes a solid body and bounds back, what is that motion called?
It is calledreflectedmotion.
At what angle does the ball leave the solid body?
It bounds off from the solid body, so that the angle of reflection is always equal to the angle of incidence, as is shown inFig. 39.
Fig. 39.
Fig. 39.
How may a stone be skipped on a smooth pond?
A stone may be skipped, by throwing it so obliquely that the water will cause it to bound off from its surface.
When a boat moves across a river at the rate of four miles an hour, and the current runs four miles an hour, in what direction will the boat go?
The boat will go diagonally across the river.
Why will the boat go diagonally across the river?
Because the boat will be carried as far down the river as the river is wide. If the river be one mile wide, then the boat, while crossing, will be carried down it one mile.
How do boatmen overcome this difficulty?
They overcome this by rowing the boat towards a point higher up the stream.
Does it take longer to cross a stream when the current is strong?
Yes; because the boat is carried farther down the stream by the current, and it must, therefore, go a greater distance in crossing.
Why is a person apt to fall when he jumps from a wagon that is moving?
Because his body partakes of the motion of the wagon and moves on after his feet are stopped by the ground, so that he falls forwards.
How can he prevent falling?
He can prevent falling by jumping in the same direction as that in which the wagon is moving.
When a stone is dropped from the top of a mast of a vessel, where does it fall?
The stone falls at the bottom of the mast.
Why does the stone fall at the bottom of the mast?
Because it partakes of the motion of the vessel, and is carried forward while falling.
When a ball is thrown perpendicularly into the air by a person on a boat, where does the ball fall?
The ball falls back into the person’s hand, although the boat may have carried him several feet while the ball was in the air.
Why does the ball fall back into his hand?
Because it partakes of the motion of the boat, and moves as far as the boat moves.
When a stone thrown from a car is aimed at a post, where will it strike?
The stone will strike ahead of the post.
How far ahead of the post will the stone strike?
It will strike as far ahead as the car moves while the stone is going to the post.
Where must we aim so as to hit a bird that is flying?
We must aim ahead of the bird.
Why must we aim ahead of the bird?
Because the bird will move a short distance through the air while the shot is reaching it.
When a body in motion strikes one at rest, is the force felt alike by both bodies?
The force is felt alike by both bodies; thus, when one head is struck against another, both are equally hurt.
Do bodies, when moving rapidly, strike with more force than when moving slowly?
Yes; the force with which they strike depends upon their velocity.
What examples of this force may be given?
A nail may be driven into a board by a hammer, when it falls rapidly upon the nail; a bullet may be shot through a plank; and even a tallow candle, when shot from a gun, may move with velocity enough to pass through a pine board.
Why do large bodies start slowly?
Because it takes some time for the force to reach all parts of them.
Why is it difficult for horses to start a loaded wagon?
Because they must pull some time before the force reaches all parts of the wagon, causing it to move.
Why is machinery slow at starting?
Because it requires time for the force applied to reach all parts of the machinery.
When two solid bodies, moving slowly, strike against each other, what takes place?
They are pushed off from each other in opposite directions.
Which body is pushed off the more rapidly?
The one having the less weight, is pushed off the more rapidly.
When the oars of a boat push against the water, what takes place?
The boat and the water move in opposite directions.
When the wings of a bird strike the air, what takes place?
The bird and the air move in opposite directions.
How can a duck or a goose swim in the water?
The toes of a duck or a goose are joined together by a web which makes the foot broad, like a paddle; and when these broad feet are pressed against the water, the duck or the goose moves forward.
Can all birds swim in the water?
No; only those birds can swim whose toes are joined together by a web, thus giving them a broad foot to be pushed against the water.
What is produced by the rubbing of bodies together?
Frictionis produced by rubbing bodies together.
What does friction tend to do?
Friction tends to check the motion of bodies.
How do we lessen the friction in machinery?
We lessen the friction in machinery by oiling the parts where they rub together.
How are waves produced in the water?
Waves are produced by the friction of the air upon the water.
What causes the water to flow in creeks and rivers?
The attraction of gravitation, which causes water always to fall to its lowest level.
What retards the current of water in streams?
The friction, caused by the water rubbing against the banks and bottoms of the streams, retards their current.
In what part of a stream is the current the strongest?
It is strongest in the middle of a stream, because there is less friction there.
What retards the flow of water through a pipe?
The friction of the water against the sides of the pipe, retards its flow very much.
Why do sudden turns in a pipe check the flow of water through it?
Because every turn in the pipe increases the friction made by the water.
Why does a wagon-wheel turn around when the wagon moves?
Because the friction between the wheel and the earth, prevents the wheel from sliding along.
Why can a steam-engine draw a train of heavy cars?
Because the friction, between the wheels and the rails, causes the wheels to turn around instead of sliding; and thus the engine moves forward drawing the cars with it.
Why is sand sometimes put on the rails?
Sand is put on the rails to increase the friction betweenthe wheels and the rails, so that the engine may start a heavier train.
Why do we use sleighs when there is snow on the ground?
Because sleighs slip easily on the snow.
Why do sleighs slip easily on the snow?
Because there is but little friction between the sleigh and the snow.
Why do wagons run more heavily when the ground is soft?
Because their wheels then sink farther into the earth, and thus the friction is increased.
Why do wagons run more heavily when the ground is covered with snow?
Because their wheels sink into the snow, and thus the friction is increased.
Why will a person slip down upon the ice more easily than upon the earth?
Because there is less friction between his feet and the ice than there is between his feet and the earth.
Why do we put ashes on our walks when they are covered with ice?
We put ashes on our walks to increase the friction between the ice and our feet, so that we may not slip down when walking on them.
Why is a lock or a brake used on a wagon?
It is used to check the motion of the wagon.
How does a lock check the motion of the wagon?
The lock is pressed against the wheels, so that enough friction is produced to check the motion of the wagon.
Why does it require more force to draw a rusty plough through the ground, than to draw one that is bright and smooth?
Because the rust increases the friction between the plough and the ground, and this makes more force necessary in order to move it.
Why cannot we hold a live eel in our hand, as easily as we can a live fish?
Because the skin of the eel is so smooth, that there is much less friction between it and our hand, than there is between the fish and our hand. Dry sand or dry ashes sprinkled on the eel will increase the friction so that it can be held.
Explorer.
Decoration.
Illustrated Capital W
What is a lever?
ALeveris a rod or bar of some kind used for raising weights.
Of what are levers made?
Levers are made of any solid substance, such as wood and iron.
When a spoon is balanced on a cup, as inFig. 40, what may the spoon be called?
The spoon may then be called a lever.
Upon what does the spoon rest?
It rests upon the edge of the cup.