Man's physical universe

xanabras

292 FORMS OF ENERGY

energy; today it is considered to be one of the best established of all

of the laws of physics. It may be stated briefly as follows:

Energy is neither created nor destroyed in any of its transformations.

The recent discovery that matter can be changed into energy and

energy into matter made it necessary to revise the law of conservation

of matter and to cease to think of matter and energy as separate

entities, for it was shown that matter is merely a form of energy.

discussion of this form of energy is

reserved for Unit VIII.

Energy Is Usually Measured in the Form of Heat.

The

Other forms of energy are more readily and more completely changed

into heat energy than into any other form; for that reason, and because

heat can be readily measured, all forms of energy are usually converted

into heat for measurement. Energy may also be measured in

terms of the work that it will do.

Heat Is Generated by Many Different Methods.

Heat may be produced by mechanical efTects,

such as friction, percussion,

and compression.

Cutting-tools often become red hot when

they are cutting iron at high speed. An emery wheel produces a

shower of red-hot sparks. Railroad rails become heated by the passage

of a train over them, while automobile brake bands sometimes catch

fire in going down steep mountains.

Babbitt metal melts out of connecting-rod

bearings when friction is increased due to inadequate

lubrication. Heat may also be produced by pounding on a nail. All

of these changes represent the transformation of mechanical energy

into heat.

The reduction of friction is one of the most important problems in

the construction of machines.

Ball bearings and roller bearings reduce

the amount of friction because they reduce the area of the surfaces of

moving objects which touch each other and substitute rolling for

slipping.

Lubricants reduce friction by forming slippery films on the

surfaces of moving objects.

to reduce friction.

Friction is often useful.

Modern motor parts are highly polished

Brakes depend upon friction for their operation.

Nails hold in wood, and automobile tires cling to the road

because of friction. Volcanoes are sometimes produced by the heat

resulting from the friction involved in diastrophism.

Heat may also be produced by changes in the composition of matter.

Every chemical reaction is accompanied by a heat change, and certain

chemical reactions constitute the chief sources of heat. When oxygen

combines with different elements, heat is evolved. Such a chemical

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