  Illustrated Capital W When is a body said to be in motion? A body is in motion when 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 in Fig. 38, what is the tendency of the ball? The tendency of the ball is to fly off in a straight line. Fig. 38. What is the force tending to make the ball fly off called? It is called centrifugal force. 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 called centripetal force. 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 called reflected motion. 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 in 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? Friction is 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 between 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. |
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