Man's physical universe



changed by the motion in

acting at once determine the path of the stone.

a horizontal direction so that both forces

It is possible to calculate just what angle to set a gun so as to cause

a given projectile to fall at a predetermined position. It can be understood

that great skill is required to hit an airplane traveling two

hundred miles per hour, with a shell from an anti-aircraft gun, because

such hits are the result of careful calculations, which must include

many factors and which must be made with little loss of time.

Newton's third law of motion simply states that:

3. To every force or action there is an equal and opposite reaction,

which is exerted on the body that exerts the action. The kickback of a

gun or a cannon is an illustration of this law. Another good example

is encountered when you stand on a freely rotating platform and turn

your head to the right; you will find that your body turns to the left.

If you wave a weight in a circular fashion above your head, your body

will rotate in the opposite direction so as to maintain a zero resultant

momentum. Now suppose that someone starts you spinning around,

and thus you acquire a given momentum. (Momentum is proportional

to the product of the mass times the speed.) If you hold out your

hands you will move more slowly because you are causing a portion

of your mass to travel through a greater distance.

Another illustration of Newton's third law of motion can be carried

out by the use of a spinning gyroscope. (A bicycle wheel, whose tire

has been replaced by many windings of heavy wire and mounted on an

axle which may be held in the hands, will serve the purpose.) If you

stand at rest on the table and hold the axle of the spinning gyroscope

horizontal, it does not affect your position. But, if you turn the axle

of the spinning wheel toward the vertical position, you will

find that

your body is spun clockwise or counterclockwise, depending upon the

direction in which the axle is tilted. The reason that you turn is that

you have imparted an angular momentum to the gyroscope, which

must be balanced by an opposite and equal angular momentum of

your body.

The principle of the gyroscope is a little difficult to understand.

The gyroscope is brought into this discussion solely because the earth,

too, is a spinning body and therefore exhibits the characteristic behavior

of the gyroscope.

If a large gyroscope is mounted on a vertical axis which is free to turn

about a horizontal axis and is kept spinning by an electric motor, it will

act as a stabilizer for monorail trains, airplanes, and ships. As the ship

rolls, it tips the axis of the gyroscope along with the rest of the ship and

thus tends to cause precession; but inasmuch as the gyroscope is so

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