# Man's physical universe

308 FORMS OF ENERGY

screw or jack is an example of a special

type of inclined plane; thus a

screw is forced into wood slowly with little power, while a nail is

forced into it quickly with more power. A jack may be used to raise a

heavy automobile because it enables a man to do a given piece of work

by exerting his

A wedge is

limited effort over a shorter distance in a given time.

a double inclined plane that enables man to increase his

force by decreasing the distance through which a force acts in a given

time.

"Give me a place to stand and rest my lever on and I can move the

This great mathematician and inventor was

man to set forth the principle of levers, which he applied in

earth," said Archimedes.

the first

constructing machines of warfare.

(A Bent

Lever)

Claw Hammer

Paper Shears

f^u- R.

- -

^^,j^^gj Barrow

Fig. 103. To which class does each of these ever>'day levers belong? (From

Foresman and Company.)

Every lever must have a bar and a pivot called the fulcrum. Some

common examples of levers are typewriter keys, automobile brake

pedals and clutches, piano keys, scissors, teeter-totters, arms, and legs.

Levers are classified as to the relative positions of the fulcrum, the

effort (force), and the resistances, as shown in Fig. 103.

The lever aids man in doing work, for it enables him to exert a great

force to move an object a short distance by doing an equal amount of

work in which his limited force is made to act through a long distance.

In addition to these three simple levers there are circular levers —

wheels, axles, and pulleys.

The ordinary cart wheel is merely a device to lessen friction and is

not classed as a machine. The wheel and axle, and pulley, on the other

hand, are used to transmit and modify force and motion.

Thus the

motion of a slowly moving large wheel of a powerful engine may be

distributed by belts, pulleys, wheels, cranks, and gears to a large number

of rapidly moving parts.

All of the marvelous machines of today are examples of various

combinations of these six simple machines — the lever, the wheel and

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