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UNIT 3
GRAVITATION
Fundamental Forces in nature
Newton’s Universal Law of
Gravitation
All objects atract
each other with a
force that is
directly
proportional to the
product of their
masses and
inversely
proportional to the
square of the
distance between
them.
 F is the force of gravity (measured in




Newtons, N)
G is the gravitational constant of the universe
and is always the same number
M is the mass of one object (measured in
kilograms, kg)
m is the mass of the other object (measured
in kilograms, kg)
r is the distance those objects are apart
(measured in meters, m)
Some features about Gravity
 Universal: The same force that makes an
object fall on Earth keeps the planets orbiting
around the Sun.
 As the Gravitational constant (G=6,67 · 10-11
N·m2/kg2) is very small, gravitational force is
only perceptible when at least one of the
objects has a great mass.
 Gravitational force is always an atractive
force.
Weight and mass
 The weight of an object is the gravitational
force acting upon the object due to the
acceleration of gravity (g).
w= m·g
 While your weight depends on where you
are, your mass has always the same value.
Activity
 Calculate your weight on the Earth, Mercury,
the Moon and Jupiter.
 You need to calculate the value of the
acceleration of gravity in each planet with the
data you will find in the next slide.
Gravitational force between
the Earth and the Sun
The centripetal force that allows the Earth to
orbit around the Sun is the Gravitational force.
Calculating orbital velocity of a satelite