GK Notes: Physics

Physics is the study of nature and its laws. The word physics has been derived from a Greek word physis which means nature. Physics is one of the academic disciplines, perhaps the oldest through its inclusion of astronomy.

Units

Measurement of any physical quantity involves comparison with a certain basic arbitrarily chosen and widely accepted reference standard called unit.

SI System

It is based on the following seven basic units and two supplementary units

Basic Units

Length

metre

Mass

kilogram

Time

second

Electric current

ampere

Thermodynamic

kelvin

temperature

Luminous intensity

candela

Amount of substance

mole

Supplementary Units

Plane angle

radian

Solid angle

steradian

Greatest Units
1 light year = 9.46 × 10 m¹⁵
1 parsec = 3.086 × 10 m¹⁶ = 3.26 ly 1 AU = 1.5 × 10 m¹¹
1 metric tonne = 10³ kg 1 quintal = 10² kg

Dimensions of Physical Quantities

Dimensions of a physical quantity are the powers, to which the fundamental quantities must be raised to represent that quantity completely. There fore, the dimensional formula of a quantity is expressed in terms of fundamental quantities, commonly mass M, length L and time T. Any physical quantity is either a scalar or a vector. e.g. Force , Density
Scalar Quantities
Physical quantities which have magnitude only and no direction are called scalar quantities. e.g. mass, speed, volume, work, time, power, energy etc.
Vector Quantities
Physical quantities which have both magnitude and direction and also obey triangle law of vector addition are called vector quantities. e.g. displacement, velocity, acceleration, force, momentum, torque etc.

Kinematics

It is the branch of mechanics, which deals with the motion of object.

Distance

▸ The length of the actual path covered by a body in a particular time interval is called distance. It is always positive.
▸ It is a scalar quantity.
▸ Its unit is metre.

Displacement

▸ The difference between the final and the initial position of an object is called displacement. It may be positive, negative or zero.
▸ It is a vector quantity. Its unit is metre.
▸ The magnitude of displacement may or may not be equal to the path length traversed by an object. Displacement ≤ Distance

Speed

▸ Speed is the distance covered by a moving body in per unit of time interval.
▸ It is a scalar quantity. It is always equal to or greater than magnitude of the velocity.
▸ The average speed of a particle for a given interval of time is defined as the ratio of total distance travelled to the total time taken. Average speed = Total distance travelled /Total time taken
▸ If the body covers first half distance with speed v₁ and next half with speed v₂, then Average speed = (2v₁ v₂)/(v₁ + v₂)

Velocity

The rate of change of displacement of a body is called velocity. Velocity = Displacement/Time
▸ Velocity is a vector quantity.
▸ It may be positive or negative.
▸ Average velocity = Total displacement/Total time
▸ If the body covers first half distance with velocity v₁ and next half with velocity v₂, then
Average velocity = (2v₁ v₂)/(v₁ + v₂)
▸ If a body travels with uniform velocity v₁ for time t₁ and with uniform velocity v₂ for time t₂, then
Average velocity = (v₁t₁ + v₂t₂)/(t₁ + t₂)
▸ If a body is moving on a circular path, then after completing one complete cycle, its average velocity is zero.
UniformVelocity
An object is said to be moving with uniform velocity if it undergoes equal displacements in equal intervals of time.
Non-UniformVelocity
An object is said to be moving with nonuniform or variable velocity if it undergoes unequal displacements in equal intervals of time.
Relative Velocity
When two bodies aremoving in the straight line, the speed (or velocity) of one with respect to another is known as its relative speed (or velocity). vAB = velocity of A with respect to B = va – vb
Acceleration
▸ It is the rate of change of velocity. Its SI unit ism/ s². It is a vector quantity.
▸ When the velocity of a body increases with time then its acceleration is positive and if velocity decreases with time then its acceleration is negative and is called retardation or deceleration.
▸ Acceleration of an object is zero, if it is at rest or moving with uniform velocity. Average acceleration,