ICSE Class 6 Physics Chapter 02 Measurement

Measurement

To give a general idea about things around us, we use terms like large, heavy, slow, and so on. However, quite often we need to say exactly how large, heavy or slow something is. To do so, we need to measure its 'largeness', 'heaviness' or 'slowness'.

We measure things every day. A grocer or a vegetable seller weighs things on a weighing scale. A tailor takes a person's measurements with a measuring tape, and then uses a scale to measure the cloth before cutting it. A petrol pump automatically measures the petrol being filled in a car. Each one of us keeps track of the time continuously with the help of a watch.

In science, a quantity that can be measured is called a physical quantity. The length of a piece of cloth, the time at which school begins and the volume of petrol are all physical quantities.

Measuring A Physical Quantity

Any measurement is a process of comparison. We compare an unknown physical quantity with a known physical quantity. The known quantity is called the unit of measurement.

Suppose we wish to measure the length of a room. Then the length of the room is the unknown quantity. We first need to choose a known standard length with which we can compare the length of the room. One metre is a known quantity. Let us choose this as our unit of length. Then we compare the length of the room with this unit. Suppose we find that this length is 5.4 times the unit. We then say that the length of the room is 5.4 metres.

In this example, the length of the room is a physical quantity. The measured value, in this case 5.4 metres, is called the magnitude of the physical quantity. Thus, the magnitude has two parts-a number (5.4) and a unit (metre). Remember: the magnitude of a physical quantity consists of a number and a unit.

Units

A unit is a known measure of a physical quantity with which physical quantities of the same kind are compared. The centimetre, metre and inch, for example, are units of length. Each of these represents a particular length. The unit we use in a measurement is a matter of choice and convenience. For example, you could choose to measure the width of a small box in centimetres or inches. But to measure the length of a curtain, you would find it more convenient to use the metre.

Traditional Units

In ancient times, different parts of the body, such as the hand or foot, were used to measure length. Obviously, measurements using such units varied from person to person. In Indian villages, you will still find distances being expressed in kos (about 3 km), and units such as kattha and bigha being used as units of land area. These units often have different values in different regions. Another unit which has different values in different regions is the gallon. One gallon represents different volumes in the UK and the USA. Units that are not clearly defined and which do not have the same value everywhere are considered nonstandard.

The use of nonstandard units leads to many problems. For example, the same experiment could yield different results in different parts of the world. To avoid such problems, we need a set or system of standard units which has the following characteristics.

1. The system of units should be used everywhere.

2. The units should be precisely defined and have the same value everywhere.

3. The units should be of convenient size. (For example, if the mile were the only unit of length, we would find it inconvenient to measure small lengths.)

The Metric System

Near the end of the 18th century, a system of standard units called the metric system was developed in France. The units for length, mass, volume and area were called the metre, gram, litre and are respectively in this system. The metric system is a logical and convenient system for the following reasons.

A single unit is defined for each physical quantity, such as length and mass.

Larger or smaller forms of a unit can be created by multiplying it by factors, such as 1000 (= 10\(^3\)) or 0.01 (= 1/10\(^2\)). In other words, the larger and smaller forms of a unit are related to each other by powers of 10.

Multiples And Submultiples Of Units

The factors for creating larger and smaller forms of a unit, called multiples and submultiples of the unit respectively, are also defined clearly. Each factor is represented by a prefix, which denotes its value. These prefixes are added before the name of the basic unit to form multiples or submultiples of the unit. Some prefixes, their symbols and the factors they represent are given in Table 2.1.

Let us consider the metre, which is the unit of length in the metric system. All lengths or distances can be expressed in terms of its multiples or submultiples. Here are a few examples.

Large distances, like the distance between towns, are measured in kilometres (km). The prefix 'kilo-' represents the factor 1000. So, the kilometre, a multiple of the metre, is 1000 times the metre (m), i.e., 1 km = 1000 m.

Smaller lengths, like the length of an eraser or the thickness of a pencil, are measured in centimetres (cm) or millimetres (mm). These units are submultiples of the metre: 1 cm = 0.01 m and 1 mm = 0.001 m.

Very small sizes, like the thickness of a hair, can be measured in micrometres (μm). One micrometre equals 0.000001 m.

SI Units

Though the metric system was adopted by many countries, scientists soon realised that it needed certain improvements. In 1960, representatives from different nations put together a system of units, based on the metric system, which is now used all over the world. It is called Systeme International d'Units (International System of Units), SI in short. This system defines the units of seven quantities, called base quantities or fundamental quantities. Of these, we will require the four given in Table 2.2.

It is enough to define the units for the base quantities because the units for other quantities can be obtained by combining the base units. For example, we can get the unit of area from its formula, which is 'length x length'. Since the unit of length is the metre, the unit of 'length x length' is 'metre x metre', written as metre\(^2\) or m\(^2\). Similarly, the unit for volume is m\(^3\).

In SI, the multiples and submultiples of units are formed as in the metric system.

Writing Units Correctly

In SI, we follow certain rules for writing units.

If a unit is named after a person, we use small letters when writing it in its full form. For example, we write kelvin, and not Kelvin. Similarly, we write watt, newton and joule, although these units are named after persons.

When we write the symbol of a unit named after a person, we use a capital letter. For example, we write K for kelvin, W for watt, N for newton and J for joule.

When we write units which are not named after persons, we use small letters for the units and their symbols. For example, metre or m, second or s.

The plural form of a unit is used when it is written in full. But an 's' is not added if the symbol of the unit is used. Thus, we write 2 metres or 2 m, but not 2 ms. Note that ms stands for millisecond.

The symbols of the prefixes are written in small letters. For example, the symbol for kilo- is k, so the symbol for kilometre is km, and not Km. K stands for kelvin.

Teacher's Note

When we buy fabric at a shop, the shopkeeper measures it in metres using a tape measure, demonstrating how we use standardized units daily to ensure fair transactions and consistent measurements.

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