Physical Quantities

Physical Quantities

  • The two main categories of physical quantities are fundamental and derived quantities.
  • Speed, area and density are examples of derived quantities.
  • Length, mass and time are examples of fundamental quantities.
  • Physical quantities are also divided in to two classes such as scalar quantities and vector quantities.
  • The quantity which has only in terms of magnitude is called scalar quantity.
  • Mass, length, time, volume, speed, energy and works are examples of scalar quantities.
  • The quantity which has in terms of both magnitude and direction is called vector quantities.
  • Velocity, momentum, force acceleration are examples of vector quantities.
  • The universal metric system of measurements recommended by the general conference of weights and measures in 1960 are called international system of units or SI units.
  • The SI unit of length is METRE abbreviated as ‘M’.
  • According to the general conference of weights and measures the ‘Metre’ is defined as the length of the path travelled by light in vaccum during a time interval of 1/299792458 of a second.
  • 1 kilometer (Km) = 1000 or m.
  • 1 milli meter (mm) = 1/1000 or m.
  • 1 centi meter (cm) = 1/100 or m.
  • The units used for measuring very small distance are :
    • a: Micro meters or microns ( )
      • 1m = µm
    • b: Angstroms ()
      • 1m =
    • c: Nanometers (nm)
      • 1m = nm
    • d:Femto meters (fm)
      • 1m = fm
  • Wave length of light is measured in Angstroms.
  • Nuclear size is measured in femtometers.
  • The distance of a star from earth or astronomical distance are expressed in light years.
  • The light year is the unit of length.
  • It is equal to the distance travelled by light in one year.
  • One light year is equal to 9.46 x
  • The unit used measuring the distance between the Earth and the Sun or the Solar System is Astronomical Unit denoted as ‘Au’.
  • Another important Astronomical unit is Parsec.
  • One Parsec is equal to 3.26 light years.
  • The ‘SI’ unit of mass is Kilogram denoted as “Kg”.
  • 1 tone is equal to kg or 1000 kg.
  • 1 gram is equal to kg or 1/1000kg.
  • 1 milligram is equal to kg or 1/1000000 kg.
  • The SI unit of time is second. It is denoted as the letter ‘s’.

Properties of Matter and Mechanics

The study of motion is said to be mechanics. When the body changes its
position with respect to something else as time goes on, the body is said to
be in motion.
The mechanical motion is of two types.
1) Translational or linear motion.
2) Rotational or Spin motion .

  • Example of Translational or linear motion the motion of a car on a road.
  • The motion of a top spinning on its axis is an example of rotational or spin motion.
  • The speed of a moving body is the rate at which it covers distance per unit of time. It is calculated by the equation as
  • Speed = Distance travelled/Time required
    • Speed is a scalar quantity.
  • SI unit of speed is meter per second denoted as m/s.
  • Velocity of an object is defined as the distance covered by an object in a specified direction in unit time interval.
  • Velocity is a Vector Quantity.
  • The SI unit of velocity is also meter per second that is m/s.
  • The major difference between speed and velocity is, speed is a scalar quantity but velocity is a vector quantity.

3) The rate of change of the velocity of a body due to change in its speed
or direction or both is said to be.


  • The acceleration is calculated by using the equation as,

Acceleration = Change in Velocity/Time

  • Acceleration is corresponding to the measures of Velocity.
  • When the velocity decreases the body is said to be declaration or retardation.
  • The most familiar acceleration is acceleration due to gravity.
  • The value of acceleration due to gravity on the surface of the earth is 9.8 .

If an object travelling with an initial velocity ‘u’ accelerates for time ‘t’
with uniform acceleration ‘a’ then the final velocity ‘v’ is given by

V = u+at

Example problem

A train travelling at 36km/hr accelerates uniformly at 2 its velocity

after ‘5’ second is
V = u + at
u = 36 Km/h or 10 m/s
a = 2
t = 5 s
There fore V = 10 + 2 x 5 = 20 m/s or 72 Km/hr.



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