Force

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Force
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Four fundamental forces
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Gravity
The gravitational force keeps you, the
atmosphere, and the seas fixed to the surface of
the planet. Though gravity is the weakest of all
interactions, it is also the least selective it
acts between all particles. Because its range is
unlimited and because it is only attractive,
gravity rules the Cosmos on a grand scale. It
holds the Earth in orbit around the Sun, keeps
the Sun locked within our galaxy of a hundred
thousand million stars, and reaches all across
the thousands of millions of galaxies that
constitute the known Universe. - E. Hecht,
Physics Calculus
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Newtons Universal Gravity
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The gravitational force on object 1 by object 2
is given by
Object 1
Object 2
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Gravitational force due to the Earth, near the
surface (weight)
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The acceleration g is approximately constant (9.8
m/s), close to the Earths surface.
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Remember constant acceleration
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What about on the surface of the moon?
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Acceleration due to gravity g does not depend on
mass
Without air
With air
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Gravitation force between the Earth and the Moon
We already calculated the centripetal
acceleration of the Moon as it orbits the
Earth. Does this agree with the gravitational
force between them, predicted by Newton?
R 3.84 x 108 m
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What if we swap the two objects?
The gravitational force on object 2 by object 1
is given by
Object 1
Object 2
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Reciprocity Newtons 3rd Law
When two bodies interact, the forces on the
bodies from each other are (1) equal in
magnitude and (2) opposite in direction.