Newtons Third Law
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-51
2025-11-01
58
Is only a statement of what we are familiar with, as reaction. If a cup is struck against the edge of a table, the table reacts against the cup and breaks it. An ocean steamship is pushed along by the reaction of water against the blades of the screw propeller. If a swimmer attempts to dive from a springboard, he makes use of both the elasticity and the reaction of the board. In order to jump far a boy must stand on something fixed, so that it shall react against the push of his muscles. If he should attempt a long jump while standing upon the seat of a swing, he would only succeed in setting the swing in motion and getting a fall.
Demonstration. - The action and reaction of elastic balls can be shown by means of a grooved board. Place a single ball in the groove and then drive a second ball against it with the end of a stick. The moving ball will stop as soon as it strikes the one that is standing still, while that one will move on with the velocity of the first.
With a half dozen balls, a number of combinations of stationary and moving balls-may be made that will be helpful in understanding the significance of this law (Fig. 1).
The "Oscillator "electric fan is an example of the practical use of reaction. In this fan a circular disk is carried at the end of a rod which is so pivoted to the support of the fan that the disk can be swung from one side of the fan to the other. When the disk is on one side, as shown, the reaction of the air upon the blades of the fan is partly counterbalanced by the blowing of the current of air upon the disk. The reaction on the other side of the fan, not being counterbalanced, pushes the blades on that side backward, and the fan support turns on a vertical axis carried by the standard. When the fan has moved a certain distance, a pin connected with the disk rod strikes a stop placed on the base, and the disk is swung to the other side of the fan. The turning of the fan is now reversed, and it moves around until the projecting pin strikes a second stop, when a reversal again takes place. In this way an automatic distribution of the air current over a room is secured.
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