Physics is the study of nature and its law. The word physics has been derived from the Greek word physis which means nature.

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

SI SYSTEM:  The International System of Units (SI, abbreviated from the French Système international)  is the modern form of the metric system. It is based on seven basic units and two supplementary units.

SI base units
Aampereelectric current
Kkelvinthermodynamic temperature
molmoleamount of substance
Cdcandelaluminous intensity
SI Supplementary Units
RadRadianplane angle
srSteradianSolid angle

Important Derived Units

SI Derived Units
QuantityDefinitionSI Unit
AreaLength Squarem2
VelocityDisplacement per unit timems−1
ForceMass & Accelerationkgms−2

FIG : The SI logo, produced by the BIPM, showing the seven SI Base Units.

Greatest Units :  

1 light year = 9.46 X 10 15 m

1 parsec = 3.086 X 10 16 m 🡪3.26 ly

1 AU = 1.5 X 10 11 m

1 metric tonne =  10 3 kg

1 quintal = 10 2 kg

Dimension of Physical Quantities :

Dimensions of physical quantity are he powers, to which the fundamental quantities must be raised to represent that quantity completely. Therefore, 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

 Scalar Quantities : Physical quantities which have magnitude only and no direction.

Ex : 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.

Ex : displacement, velocity, acceleration, force, momentum, torque etc


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


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


  • The difference between the final and the actual position of an object. It may be positive, negative or zero. 
  • It is a vector quantity
  • The magnitude of displacement may or may not be equal to the path length traversed by an object.
  • [Displacement] [Distance].


  • Distance covered by a moving body in per unit of time interval.
  •  It is always equal or greater than magnitude of the velocity.
  • It is a scalar quantity.
  • 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.

Total Distance Travelled 

Average Speed = ————————————

    Total Time Taken

  • If the body covers first half distance with the speed v1 and the next half with speed v2, then


Average Speed = —————————

    v1 + v2


  • The rate of change of displacement of a body.


Velocity = —————————-


  • Velocity if a  vector quantity.
  • It may be positive or negative

 Total Displacement 

Average Velocity = —————————-

                Total Time

  • If the body covers first half distance with the speed v1 and the next half with speed v2, then


Average Velocity = ———————

    v1 + v2

  • If a body travels with uniform velocity v1 for the time t1, and with  uniform velocity v2 for the time t2, then 

     v1t1 + v2t2 

Average Velocity = ———————

          t1 + t2

  • If a body is moving on a circular path then after completing one complete cycle, its average velocity is zero

Uniform Velocity

  • An object is said to be moving with uniform velocity if it undergoes equal displacements in equal intervals of time.

Non – Uniform Velocity

  • An object is said to be moving with non-uniform or variable velocity if it undergoes unequal displacements in equal intervals of time.

Relative Velocity

  • When two bodies are moving in the straight line, the speed (or Velocity) of one with respect to another.

vAB = velocity of A with respect to B = vA – vB


  • It is the rate of change of velocity.
  • SI Unit is m/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 if negative and is called retardation or deceleration.
  • Acceleration of an object is zero, if it is at rest or moving with uniform velocity.

     v1 – v2 

Average acceleration  α = —————-



= ————



  • If the position of a body or a system of bodies does not change with time , it is said to be at rest.
  • On the other hand if the position change with respect to time, it is said to be in motion.
  • A particle in rest does not have the speed and acceleration, while a particle in the motion has its speed and also may have some acceleration, if the speed changes with respect to time.

Equation of Motion

For a motion on a straight line with constant acceleration α

  1. v = u + αt
  2. ut = ½ at2
  3. v2 = u2 + 2 αs.

Equation of Motion Under Gravity :

  1. Downward Direction
  1. v = u + gt
  2. h = ut +  ½ gt2
  3. v2 = u2 + 2 gh

Where,  s 🡪  displacement  travelled,  h 🡪 height,  t 🡪 time,   u 🡪 Initial velocity,      

              v 🡪 final velocity, α 🡪 acceleration, g 🡪 acceleration due to gravity

Note : for retardation α will be replaced by – α

  1. Upward Direction : if velocity of a body is decreasing instead of increasing, then the equation of motion are
  1. v = u – gt
  2. h = ut –  ½ gt2
  3. v2 = u2 – 2 gh
  1. Distance travelled by a body in nth second snth

  snth = u + (2n – 1)  α / 2If the body is thrown upwards, then it will rise until its vertical velocity becomes zero. Then the maximum height attained is h = u2 / 2g.

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