```Center of Gravity                                       Satellite
It is the point in a body around which the                 A satellite is a moon, planet or machine
resultant torque due to gravity forces vanish.              that orbits a planet or star. For example,
Near the surface of the earth, where the gravity            Earth is a satellite because it orbits the
acts downward as a parallel force field, the                sun. Likewise, the moon is a satellite
center of gravity and the center of mass are the            because it orbits Earth.
same.                                                      Usually, the word "satellite" refers to a
machine that is launched into space and
Mass (m)                                                    moves around Earth or another body in
space.
    It is the quantity of matter in a body                The world"s first artificial satellite, the
regardless of its volume or of any forces              Sputnik 1, was launched by the Soviet
acting on it.                                          Union in Oct. 4, 1957.
    It should not be confused with weight,                The path a satellite follows is an orbit. In
which is the measure of the force of                   the orbit, the farthest point from Earth is
gravity acting on a body.                              the apogee, and the nearest point is the
    Its unit is the kilogram (kg).                         perigee.
    mass = Force / acceleration [m = F/a ]
    mass = momentum / velocity [ m = p/v ]             Important types of Satellite
    mass = weight / acceleration of gravity [
   Anti-Satellite             weapons/"Killer
m = W/g ]
Satellites" : to destroy enemy warheads,
satellites, and other space assets.
Weight (W)                                                 Astronomical satellites : observation of
distant planets, galaxies, and other outer
    It is the force exerted on a body by gravity           space objects.
and calculated as the mass times the                  Biosatellites : to carry living organisms,
acceleration of gravity, w = mg.                       generally for scientific experimentation.
    Its unit is the newton (N).                           Communications           satellites        :
    For an object in free fall, so that gravity is         telecommunications
the only force acting on it, then the                 Navigational satellites : use radio time
expression for weight follows from                     signals transmitted to enable mobile
Newton"s second law.                                   receivers on the ground to determine their
    W = mg                                                 exact location.
    Weight of object = mass of object x                   Reconnaissance satellites           : Earth
acceleration of gravity                                observation satellite or communications
satellite deployed for military or
Weight of a PERSON inside the Lift                          intelligence applications.
   Earth observation satellites         : non-
military uses such as environmental
monitoring, meteorology, map making etc.
   Weather satellites : used to monitor
Earth"s weather and climate.
   Manned spacecraft (spaceships) : large
satellites able to put humans into (and
beyond) an orbit, and return them to Earth.
   Space stations         : man-made orbital
structures that are designed for human
beings to live on in outer space.
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