**Physical quantity**

In day-to-day life, we measure many things such as the weight of fruits, vegetables, food grains, temperature of the body or some liquids, volume of liquids, density of various substances, the speed of vehicles, etc. Quantities such as mass, weight, distance, speed, temperature, volume are called physical quantities. A value and a unit are used to express the magnitude of a physical quantity. For example, Swarali walks two kilometres everyday. In this example, ‘two’ is the value and ‘kilometre’ is the unit used to express the magnitude of the distance which is a physical quantity

**Mass**

The amount of matter present in a substance is called mass. Matter has a natural tendency to resist a change in its state, which is called inertia. Mass is the qualitative measure of the inertia of an object. The larger the mass, the greater is the inertia. Mass is a scalar quantity. It does not change from place to place anywhere in the world. The quantities mass and weight are, however, different. Gram and kilogram are the units of mass. When we use the two-pan common balance in a shop, we compare two masses.

**Weight**

What we measure in grams, kilograms is mass, and not weight. The gravitational force that acts on this mass is called its weight. The gravitational force by which the earth attracts an object towards its centre is called the weight of the object. Therefore, weight is a vector quantity. It is different at different places on the earth.

Will it be possible to use one and the same unit to measure physical quantities such as mass, weight, distance, velocity, temperature ? In everyday affairs, we measure many different physical quantities. As these physical quantities are different from each other a specific unit is used to measure each quantity. Therefore, different units are used while measuring different quantities.

**Standardized measurement**

- Take a ball of string. Let one student from the class measure four hand-spans of the string and cut it there. Let each of the other students in the class cut four hand-spans of the string, too. Now hold all the pieces together by one end. Are they all of the same length?
- Now, measure the length of a bench by means of the span of your hand. Ask your friends to do the same. Did each of you obtain the same measure for that bench? What could be the reason ?

Standardized measures are required for measuring things. Such measures are called standard units.

We have to measure many physical quantities accurately. To measure any quantity, we use the unit specified for it.

For example, the metre (m) is the specified unit for measuring length. A certain distance has been accepted as the standard for 1.0 metre. Why is there a need for such a standard unit ? Suppose, the span of a hand is accepted as the unit for measuring length. With this unit, we can measure lengths of cloth as two hand-spans, three hand[1]spans, and so on. However, the lengths of the cloth measured by each one of us will come out to be different. That is why a ‘hand-span’ cannot be a standard unit for measuring length.

There are many physical quantities but a majority of them are related to each other. For example, you have learnt that the quantity ‘speed’ is the ratio of the quantities ‘distance’ and ‘time’.

**Fundamental quantities :**

It is enough to select a few out of the many quantities and standardize their units. You can see from the above examples, that units of the quantities length and time need to be standardized. Such quantities are called fundamental quantities and their units are called standard units. Of course, a standard fundamental unit must be available to all, and it must not be variable.

**International system of units :**

An international system of units based on seven fundamental units, called the System International (SI), is currently used all over the world. It is also called the metric system. The names and symbols of the units of the fundamental quantities, length, time and mass, in this system, are given in the following table :

**Importance of accurate measurement**

How accurate a measurement must be depends upon its purpose. Accordingly, an appropriate device has to be used for the measurement. Measurement of substances that are precious, of great importance and used in very small quantities, is done meticulously and accurately. Due to advancements in technology, devices that measure very small magnitudes of quantities like distance, mass, time and temperature, are available now, for example, distance and time in connection with very important sports competitions, mass of gold, body temperature, etc.

**Major causes of errors in measurement**

- Not using the appropriate device.

2. Not using the device properly.

Make a list of possible errors other than these. Do you really get as many litres of petrol as the petrol pump indicates ? To ensure this, it is necessary to check it against a standard measure from time to time. This is called standardization. Similarly, it is necessary to standardize the weights and measures used in the market. While buying things at grocery shops, the vegetable market, remember to look out for the following and tell your guardians to do so, too.

- Does the balance carry the stamp of standardisation by the department of weights and measures ?

2. Is the balance stable ? Is the pointer of the balance upright ?

3. Is the weight made of metal ? How is the balance held?

4. Has the underside of the pan of the balance been tampered with?