Physical quantities and units of measurements

Physics is a science based upon exact measurement, so you must be familiar with commonly used measuring devices and the units of measurements.

There are three basic concepts: length, mass and time. The units used to measure them are called fundamental units. All other units are called derived units because they can always be written as some combination of the three fundamental units.

To take a few examples

area = length x length

volume = length x length x length

speed = length./time

density = mass/length x length x length

There are two widely used sets of fundamental units: (a) the Metric System and (b) the English System.

The Metric System or the International decimal system of weights and measures is based on the metre and kilogramme. Using metric units distance (length) is usually measured in millimetres, centimetres, metres or kilometres; time is measured in seconds, minutes, or hours; and mass is measured in grammes or kilogrammes.

The English System uses the foot, yard and mile as units of length; the ounce, pound and ton as units of force and the second as the unit of time.

The chief advantage of the Metric system over the English units is that all metric units are divided into 10 or 100 parts. This enables fractional distances and masses to be expressed as decimals. Decimals, it is well known, are easier to manipulate in the addition, subtraction, multipli­cation, and division of two or more quantities.

The Foot-Pound-Second (F.P.S.) System is used in Great Britain and the United States of America. The Metric System (metre-kilogramme-second) is invented in France and accepted universally in science but not in engineering or commerce.

Engineers and scientists have produced a code of standard symbols for convenient representation of physical quantities.

This is a list of standard symbols*.

length — l work — w

mass — m power — P

time — t

area — A electric potential — V

volume — V electric current — I

velocity -v electric resistance –R

acceleration — a

temperature — t

density — p heat — Q

force — f specific heat — c

moment — m latent heat — l

pressure — p

Date: 2015-09-02; view: 2947; Нарушение авторских прав