Measurements and Units

Systems of Units. — Some measurements are fundamental and seem to refer to concepts that cannot be further analyzed. There are three of these basic concepts: time, space, and mass. The units used to measure them are called fundamental units. There are other magni­tudes that cannot be thought of without connecting them with two or more of these fundamental units more than once. Units of this kind that involve more than one fundamental unit, or one fundamental unit more than once, are called derived units. Units of speed, units of vol­ume, units of area, units of acceleration, etc., are derived units.

Units of Time. —For many scientific purposes the second is chosen as the unit of time. It is defined as the 1/86,400 part of the mean so­lar day. The minute, which is equivalent to 60 sec, and the hour, which is equal to 60 min, are also used as units of time.

Measurement of Angles. —Degrees, minutes, and seconds are the familiar units in which angles are measured. A degree is the angle subtended at the center of a circle by an arc equal to one three hundred sixtieth part of the circumference of the circle. A minute is one sixtieth of a degree and a second is one sixtieth of a minute. There are, therefore, 360 deg in a circle, 60 min in a degree, and 60 sec in a minute.

The angular unit ordinarily used in physics is called a radian.

The two systems of units commonly employed are the decimal met­ric system devised by the French and the more familiar English system [2, C. 39-40].

2.1.3 Look through texts 2.1.1 - 2.1.2 and find the English equivalents for the following Russian phrases and word-combinations:

истинная область изучения физики; взаимопроникающие области; производные единицы; единицы ускорения; обычно применяемые единицы; десятичная метрическая система.