Topic 4

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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 meter and kilogram. Using metric units distance (length) is usually measured in millimeters, centimeters, meters or kilometers; time is measured in seconds, minutes or hours; and mass is measured in grams or kilograms.

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 (meter-kilogram-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 pressure – p

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

To simplify the representation of units a standard list of abbreviations has been adopted.

Here is a list of common abbreviations. Learn them by heart:

foot – ft liter – l

pound – lb Newton – n

second – s kilogram force – kgf

gallon – gal watt – w

pound force – lbf volt – v

radian – rad kilogram calorie – kcal

horsepower – hp Temperature rise (Fahrenheit) – deg F

ampere – A Temperature rise (Celsius) – deg C

meter – m Coulomb – C

gram – g British thermal unit – Btu

kilogram – kg Celsius heat unit – Chu

minute – min Metric horse power - Ps

In practice units of one system may be converted to units of the other system.

Exercises:

1 Read and translate the following words and word-combinations:

1.1 a science

1.2 exact measurement

1.3 measuring devices

1.4 to measure

1.5 fundamental units

1.6 derived units

1.7 chief advantage

1.8 to be invented

1.9 to be accepted universally in science

1.10 to produce a code of standard symbols

1.11 convenient representation of physical quantities

1.12 to simplify

1.13 to adopt a list of abbreviations

1.14 to be familiar with

1.15 to manipulate

2 Translate into English:

длина, масса, время, площадь, объём, скорость, плотность, вес, метр, килограмм, фут, ярд,

миля, унция, фунт, тонна, сила, мощность, напряжение, ток, сопротивление, ускорение,

давление, температура, теплота.

3 Make up word-combinations from the following columns:

3.1 basic system

3.2 physical representation

3.3 decimal abbreviations

3.4 fractional concepts

3.5 to simplify as decimals

3.6 common units

3.7 to convert distances

3.8 to be expressed quantities

4 Transcribe the following words:

quantity, measurement, advantage, scientist, engineer, pressure, acceleration, addition, subtraction,

multiplication, division, commerce, ounce.

5 Complete the sentences:

5.1 Physics is a science …

5.2 The Metric System or the International decimal system of weights and measures is …

5.3 The English System uses …

5.4 The chief advantage of the Metric system over the English units is that …

5.5 Engineers and scientists have produced …

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