Maharashtra Board Class 7 Science Chapter 6 Measurement of Physical Quantities PDF Download

6. Measurement Of Physical Quantities

How are the various articles and materials, shown in the picture measured?

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.

Scalar Quantity

A quantity that can be completely expressed by its magnitude alone is called a scalar quantity. For example, only magnitude, i.e. a value with a unit, is used to express quantities such as length, breadth, area, mass, temperature, density, time, work, etc. Thus, we say that the length of a tunnel is two kilometres, the fever (temperature) is 101° Fahrenheit, etc.

Vector Quantity

The quantity that is expressed completely only when magnitude and direction are both given is called a vector quantity.

Displacement, velocity are vector quantities. For example, a displacement of 20 kilometres towards the north, the aeroplane flying at a velocity of 500 km/hr towards Mumbai.

Teacher's Note

In our daily life, we use these quantities. For example, when we go to the market, we buy 2 kg of vegetables. Here 2 kg is the mass.

Exam Trick

Remember: Scalar = only size, like 5 kg of rice. Vector = size and direction, like 5 km towards north.

Points to Remember

Physical quantity needs a value and a unit to be measured.
Scalar quantity has only magnitude.
Vector quantity has both magnitude and direction.
Mass, temperature, and area are scalar quantities.
Displacement and velocity are vector quantities.

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.

Teacher's Note

Your weight changes on the moon because gravity is different there. But your mass stays the same everywhere. It is like your name, which stays the same at home and at school.

Exam Trick

Remember: Mass = same everywhere (like your Aadhaar number). Weight = changes with gravity (like your pocket money that changes at home and at grandma's house).

Points to Remember

Mass is the amount of matter in an object.
Weight is the force of gravity pulling on an object.
Mass is the same everywhere in the world.
Weight changes from place to place.
Mass is a scalar quantity and weight is a vector quantity.

1. Why would the weight of an object be maximum at the poles and minimum at the equator?

2. Why is the weight of an object at a high altitude less than its weight at the sea-level?

Our body has weight because of the gravitational force of the earth. The gravitational force of the moon being less, our weight will turn out to be less there. Our mass, however, is the same at both the places.

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

1. 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?

2. 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-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.

Teacher's Note

Standard units help everyone measure the same way. Like how all schools follow the same height and weight chart for students, so doctors can compare properly.

Exam Trick

Remember: Standard unit = same for everyone. Hand-span = different for each person. That is why we use metres, not hand-spans.

Points to Remember

Every person's hand-span is different in size.
We need standard units so everyone measures the same way.
The metre is the standard unit for measuring length.
Standard units do not change from person to person.
Without standard units, different people get different measurements.

Prevailing Systems Of Measurement

1. MKS System : In this system distance is measured in metres, mass in kilograms and time in seconds.

2. CGS System : In this system distance is measured in centimetres, mass in grams and time in seconds.

In the MKS system of measurement, distance, mass, and time are accepted as the fundamental quantities. These three quantities are used to measure all other quantities.

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:

Standards Of The Fundamental Quantities

As the standard of mass a solid cylinder made of a platinum-iridium alloy is kept in the International Bureau of Weights and Measures at Paris. As per the international agreement, this mass is called one kilogram. Official accurate copies of this prototype are kept in the standardizing laboratories/institutes all over the world.

Two fine lines are engraved on a platinum-iridium bar kept in the International Bureau of Weights and Measures at Paris. The distance between these two lines is accepted as the standard metre. Accurate copies of this prototype are made and distributed to standardizing laboratories/institutes all over the world.

The time required for one revolution of the earth is measured by means of an accurate device. This time is taken as 24 hours. To standardize one second, one hour is considered to have 60 minutes and one minute, 60 seconds.

Teacher's Note

In Paris, France, there is a special bar made of platinum and iridium. The distance between two lines on this bar is the official standard for one metre. All other metres in the world are copies of this.

Exam Trick

Remember: Standard metre is in Paris. Standard kilogram is also in Paris. These are like the "originals" and all other measurements are "photocopies" of them.

Points to Remember

The standard metre is kept in Paris, France.
The standard kilogram is a platinum-iridium cylinder kept in Paris.
One hour has 60 minutes and one minute has 60 seconds.
SI is the international system used all over the world.
Metric system is another name for the SI system.

1. What is an atomic clock? Where is it kept?

2. How is the velocity of light used for determining the standard metre?

When man felt the need to measure things, he started by using the parts of his own body for the purpose. In Egypt in ancient times, the distance from a man's elbow to the tip of his middle finger was called a 'cubit'. This measure would differ from person to person. Therefore, the cubit of the King was considered as the standard. In olden times, we weighed gold in a unit called gunj. An 'hour glass' was used for measuring time. Have you seen one?

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