Physical quantities : 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. are measured by their physical quantities.

From above the quantities such as mass, weight, distance, speed, temperature, volume etc. are called Physical quantities.

Magnitude of a physical quantity : To express the magnitude of a physical quantity, a value and a unit are used. e.g.,

For measuring the distance metre and kilometre are the units that are used. If the distance between two cities is 120 kilometres, then 120 is the value and kilometres is the unit of the distance. Different physical quantities are measured by different units.

Scalar Quantity : A scalar quantity is completely expressed by its magnitude alone

e.g., To express quantities such as length, breadth, area, mass, temperature, density time, work, etc. value with a unit is used. e .g., length of a road : 5 kilometres, Temperature of a sick person : 102°Farenheit etc.

Vector quantity: Scalar quantity is expressed completely with both magnitude and direction. e.g., Displacement velocity e.g., A displacement of 22 kilometres towards the, North, the aeroplane flying at a velocity of 450 km/hr towards Delhi.

Mass : The amount of matter present in a substance is called mass.

• Matter has inertia which is a natural tendency to resist a change in its state.
• Mass is the qualitative measure of the inertia of an object. This means that if the mass is larger then the inertia is greater.
• Mass is a scalar quantity. Units of mass are gram and kilogram.

Weight : Weight is the gravitational force that acts on this mass.

• The weight of the object is actually the gravitational force by which the earth attracts that object towards its centre.
• Weight is a vector quantity: Weight differs at different places on the earth.
• The unit of weigh in SI system is Newton (N).

The gravitational force of the moon is lesser as compared to the gravitational force of the earth. Therefore the weight of a body is less on the moon. However, mass remains the same on moon as well as on earth.

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

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

Standardized measurement : It is not scientifically correct to measure the physical quantities without any standard method. Hence to keep uniformity of measurement, a standardized measurement methods have to be followed. For accurate and standard measurement, every physical quantity is measured in a standard and specific unit.

e.g., Length is always measured in unit of metre (m).

Fundamental quantities :

• The quantities for which the standardized units can be decided are called fundamental quantities. e.g., the distance and time are the quantities for which standard units have been decided.
• There is fundamental unit for each fundamental quantity. This fundamental unit is available to all. There are seven basic fundamental quantities.

Standards of the fundamental quantities :

• At the International Bureau of Weights and Measures at Paris, the standard of mass a solid cylinder made of a platinum-iridium alloy is kept. The mass of this cylinder is one kilogram as per the international agreement.
• At the same place, platinum-iridium bar is kept. It has two fine lines engraved on it.
• Official accurate copies of these prototypes are kept in the standardizing laboratories/institutes all over the world
• The time required for one revolution of the earth is taken as 24 hours. It is measured by an accurate device. This is considered to be one day. One hour is considered to have 60 minutes and one minute to have 60 seconds From this one second is standardized.

Atomic clock : Atomic clock is a device that works with utmost accuracy. Its work is dependent on the vibrations of atoms and molecules. The functioning is due to molecular systems of Caesium or ammonia. The atomic clock in Mumbai is in National Physical Laboratory in New Delhi.

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

Major causes of errors in measurement :

• Errors occur if the appropriate device is not used.
• Also if the device is not used properly, errors do occur in measurement.

Standardization : It is necessary to standardize the weights and measures used in the market. Therefore, standard measures are checked from time to time by the Government Department.