NCERT Solutions For Class 9 Science Book

Class 9 Science Physics is all about understanding our physical environment. The laws of physics explain the various phenomena of nature. These laws are based upon observation & experiment. Careful observations & constant experimentation forum the basis of every principle in physics. Experiment require measurements. Hence physics can be called as the science of measurements

1) PHYSICAL QUANTITIES:The quantities which can be measured by an instrument & by means of which we can describe the laws of physics are called physical quantities. In other words, physical quantities are any quantity that can be defined and measured E.g.: Length, mass, velocity, force, density, time etc.

There are large numbers of physical quantities and every physical quantity needs a unit to be measured. However, not all quantities are independent E.g.: If a unit of length is defined, the unit of volume is automatically obtained. Thus we can define a set of fundamental quantities and all other quantities can be expressed in term of the fundamental quantities

2) UNITS:Measurement of any physical quantity is express in terms of an internationally accepted certain basic standard called as unit. Unit is the quantity of a constant magnitude which is used to measure the magnitude of other quantities of the same nature.

E.g.: Unit of length is metre, and of time is second, etc.

A physical quantity must be expressed in the form of magnitude and unit. Thus, the magnitude of a physical quantity is expressed as below:Physical Quantity = (numerical value) ´ Unit Eg: If the length of a piece of cloth is 10 metre, it means that the length is measured in the unit metre and this unit is contained 10 times in the length of that piece of cloth

3) TYPES OF PHYSICAL QUANTITIES & THEIR UNITS: There are two types of physical quantities They are: 1) Fundamental quantities 2) Derived quantities 1)

Fundamental Quantities: Fundamental quantities are those which cannot be expressed in terms of other quantities. It is independent of any other quantity it can neither be changed nor related to any other fundamental quantity.

E.g.: Mass, length, Time, Temperature, Current, Amount of substance, Angle, Luminous Intensity.

2.

Derived Quantities:

Derived quantities are those which depend on other fundamental quantity or which can be expressed in terms of other quantities E.g.: Velocity, force, acceleration, etc.

4) Systems of units In olden days, measurement started with the barter system, which consisted of the exchange of one article for another. Later on, usage of parts of the body came into existence. For example, feet, palms arm length were used as standards for the measurement of length. Finally, scientific units were developed.

In today’s world, based upon the units of fundamental quantities (length, mass and time), there systems of units are prevalent.

5) Types of systems of units: 1) FPS of the British system 2) CGS or the Metric system 3) MKS or the SI system The systems have been named on the basis of the units for the fundamental physical quantities length, mass and time

Metre

F

Foot

C

Centimetre

M

P

Pound

G

Gram

K

Kilogram

S

Second

S

Second

S

Second

6) Fundamental quantities in SI system: The fundamental quantities, their units and their respective symbols in the SI system, are given below.