Experiment 1. -Pull out the handle of a compression air-pump or bicycle pump. Close the exit valve or stop up the end of the bicycle pump. Now try to push in the handle. What keeps it from moving easily? Try to shove an inverted drinking glass into a pail of water. Why does not the water fill the glass?
All matter as we know it occupies room or space. In other words, it has extension. When we pump up a bicycle tire we find that even the air demands room for itself. In the experiment with the air compressor we found that the space occupied by the air could be reduced only to a limited extent. However great the pressure might have been the air would still have occupied a certain amount of space.
Experiment 2. -Place a coin on a card extending slightly beyond the edge of a table. Suddenly snap the card horizontally. Does the coin move?
Fig. 1.
Another of our common observations is that a body does not begin to move unless some force acts upon it, nor when moving does it stop unless some force stops it. When the card was snapped from under the coin, the coin did not appear to move because the friction of the paper was not sufficient to transfer any appreciable motion to it. If the coin had been glued to the card, both coin and card would have moved.
Experiment 3. -Revolve around the hand a small weight attached to a strong rubber band. Suddenly let go the band. Does the weight keep on moving in the circular path in which it was revolving?
When a car is moving along a level track we do not expect it to stop until the friction of the trackor some other force stops it. When we revolvedFig. 2.
the weight attached to the rubber band and let go the band the weight started off in a straight line. It did not continue in this straight line because a force, gravity, pulled it down toward the earth. This property which bodies have of remaining at rest unless acted upon by some force, and when in motion of continuing to move in astraight line with the same speed unless acted upon by an outside force, is called inertia. Sir Isaac Newton first stated this fact, and so it is sometimes called Newton"s First Law. It is due to inertia that people are thrown out of an automobile if it is suddenly stopped.
Experiment 4. -Suspend a heavy ball by a string not much too strong to hold it. (Place a pad beneath it to catch it if it drops.) Attach a similar string to the bottom of the ball. Attempt to lift it suddenly by the upper string. What happens? Suspend it again and pull down gradually on the lower string. What happens? Suspend it again and pull down suddenly on the lower string. What happens?
Fig.3.
When we tried suddenly to lift the suspended ball the force of inertia was so great that it broke the string. When the string was attached to the bottom of the ball and the pull gradually exerted, the upper string broke, since it had both the weight of the ball and the pull of the string to withstand; but when the pull was suddenly exerted, the inertia of the ball was sufficient to withstand the pull, and the lower string broke.
It is the inertia of the water which enables the small, rapidly revolving propeller to move the big ship. The same is true of both the propelling and supporting of flying machines. The resistance which the particles of air offer to being suddenly thrown into motion, their inertia, enables the propeller to push the aeroplane along and keeps it from falling to the ground as long as it is moving rapidly. It is inertia which keeps the heavenly bodies moving in space. Once in motion they must keep on forever unless some force stops them.
A BIPLANE.
The blur shows how swiftly the propeller is revolving.
Experiment 5. -Place a glass globe partly filled with water on a rotating apparatus. Rotate the globe rapidly. What does the water tend to do?
Inertia also manifests itself in the tendency of revolving bodies to move away from the centerFig.4.
around which they are revolving. Inertia thus manifesting itself is calledcentrifugal force. An example of this was seen in Experiments 3 and 5.
Newton many years ago discovered that all bodies of matter have an attraction for each other and that this force of attraction varies as the masses of the bodies, that is, the more matter two bodies contain the more they attract each other. But this attraction becomes less as the distance between the bodies increases. This lessening of the force of attraction on account of the increase of distance is proportional not to the distance, but to the square of the distance. This means that the attraction between the same bodies when twice as far apart is only one fourth as great; when three times as far apart, one ninth as great, and so on. What causes this attraction no one knows, but the name givento this force of attraction is Gravitation. Gravitation is always acting upon all bodies, and their conduct is constantly affected by it. It keeps the heavenly bodies from wandering away from each other just as the rubber band kept the weight from flying away from the hand.
When this attraction is considered in relation to the earth and bodies near its surface the term gravity is used. We are constantly measuring the pull of gravity and calling it weight. This is the cause of bodies falling to the earth. It is the force which causes us to lie down when we wish to sleep comfortably, and frequently makes men fall who try to fly.
If two forces act upon a free body, each will influence the direction of its motion and it will go in the direction of neither force, but in a direction between the two. If there are more than two forces, the path will be the result of the action of all the forces. In the case of the weight and the rubber band we found that the moving weight when notheld by the force of the band flew away from the hand. The rubber band continually pulled it toward the hand. The result of these two forces, the "centrifugal force" andTHREE FORCES IN PLAY.