Power (P)                                                     1 W = 1 J/s
                                                              1 kW = 103 W
    Rate of doing work by a body is called                   1 MW = 106 W
     power (P).                                               1 Horse Power = 746 W
     Power (P) 
                  WorkDone(W)                                 1 watt second (W-s) = 1 J
                  Time Taken(t)                               1 watt hour (W-h) = 3600 J
    The unit of power is the joule per second                1 kilowatt hour (kW-h) = 3.6 x 106 J
     (J/s), known as the watt (in honor of
     James Watt, the eighteenth-century
     developer of the Steam Engine).
Newton"s Law of Universal Gravitation:                     Gravity (g)
Gravitational Force
                                                              It is the force that attracts a body toward
Isaac Newton discovered in the 17th century                    the center of the earth, or toward any other
that the same interaction that makes an apple                  physical body having mass.
fall from a tree also keeps planets in orbit                  The gravity of Earth, denoted g, refers to
around the sun. Along with his three laws of                   the acceleration that the Earth imparts to
motion, Newton published the law of                            objects on or near its surface.
gravitation in 1687. It can be stated as follows:             Its unit is meters per second squared (in
                                                               symbols, m/s2 or m·s−2) or equivalently in
“Every particle of matter in the universe                      newtons per kilogram (N/kg or N·kg−1).
attracts every other particle with a force that               It has an approximate value of 9.8 m/s2,
is directly proportional to the product of the                 which means that, ignoring the effects of
masses of the particles and inversely                          air resistance, the speed of an object
proportional to the square of the distance                     falling freely near the Earth"s surface will
between them.”                                                 increase by about 9.8 meters (about 32.2
                                                               ft) per second every second.
In mathematical terms, the law of universal
gravitation may be given by                                So, to find the acceleration due to gravity at
                                                           sea level, substitute the values of the
       m1 m2                                               gravitational constant, G, the Earth"s mass (in
          r2                                               kilograms), m1, and the Earth"s radius (in
                                                           metres), r, to obtain the value of g:
                                                                 m1                      5.9736  1024
                                                           gG        (6.6742  1011)                    9.822m.s2
F is the magnitude of the gravitational force                    r 2
                                                                                        (6.37101  106 )2
between the two point masses,
G      is    the    gravitational      constant   or       Variation in gravity (g)
 G  (6.67428  0.00067)  1011 m3 kg1 s2
                                                              Acceleration due to gravity decreases with
m1 is the mass of the first point mass,                        increase in height/altitude.
                                                              g is maximum at poles.
m2 is the mass of the second point mass,                      g is minimum at equator.
                                                              g decreases due to rotation of earth.
r is the distance between the two point masses.               g decreases if angular speed of earth
                                                               increases and increases in angular speed of
                                                               earth decreases.