# Measurements

## Mass

Mass  Earth has a mass of 5.98 X 1024 kg. The average mass of the atoms that make up Earth is 40 u. How many atoms are there in Earth’s? 20 . Gold, which has a mass of 19.32 g for each cubic centimeter of volume, is the most ductile metal and can be pressed into a …

## Time

Time 10E. Physicist Enrico Fermi once pointed out that a standard lecture period (50 Olin) is close to I microcentury. (a) How long is a rnicrocentury in minutes? (b) Using find the percentage difference from Fermi’s approximation. liE. Express the speed of light, 3.0 X 108 m/s, in (a) feet per nanosecond and (b) millimeters per picosecond. 12E. …

## length

length IE. The micrometer (I /Lm) is often called the micron. (a) How many microns make up 1.0 km? (b) What fraction of a centimeter equals 1.0 J.U!!?(c) How many microns are in 1.0 yd? 2E. Two types of barrel units were in use in the) 920s  in the United States. The apple barrel had a legally set …

## Mass

Mass The unit of mass-the kilogram-is defined in terms of a particular platinum-iridium prototype kept near Paris, France. For measurements on an atomic scale. the atomic mass unit, defined in terms of the atom carbon-12, is usually used.

## Time

Time The unit of time-the second-was formerly defined in terms of the rotation of Earth. It is now defined in terms of the oscillations of light emitted by an atomic (cesium-133) source. Accurate time signals are sent worldwide by radio signals keyed to atomic clocks in standardizing laboratories.

## Length

Length The unit of length-the meter-is defined as the distance traveled by light during a precisely specified time interval.

## Changing Units

Changing Units Conversion of units from one system to an other (for example. from miles per hour to kilometers per second)may be  performed by using chain-link conversions in which the original data are multiplied successively by conversion factors written as unity and the units are manipulated like algebraic quantities until only the desired units remain.

## SI Units

SI Units The unit system emphasized in this book is the International System of Units (SI). The three physical quantities displayed in Table I-I are used in the early chapters. Standards, which must be both accessible and invariable. have been established for these base quantities by international agreement. These standards are used in all physical measurement, for both the base …

## Measurement in Physics

Measurement in Physics Physics is based on measurement of physical quantities. Certain physical quantities have been chosen as base quantities (such as length, time, and mass); each has been defined in terms of a standard and given a unit of measure (such as meter, second, and kilogram). Other physical quantities are defined in terms of the base quantities and their …

## Mass

Mass  The Standard Kilogram The SI standard of mass is a platinum-iridium cylinder (Fig. 1-4) kept at the International Bureau of Weights and Measures near Paris and assigned, by international agreement, a mass of 1 kilogram. Accurate copies have been sent to standardizing laboratories in other countries, and the masses of other bodies can be determined by balancing them …