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Newtons Law of Gravitation
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Gravity is More Than a Name
Gravity is the name associated with the
reason for what goes up, must come
down
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We all know of the word gravity- it is
the thing which causes objects to fall to
Earth.
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ROLE of PHYSICS
To do more than to associate words with
phenomenon.
To explain phenomenon in terms of underlying
principles
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A students conception of gravity must grow in
sophistication to the point that it becomes more
than a mere name associated with falling
phenomenon
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Gravity must be understood in terms of its
cause, its source, and its far-reaching
implications on the structure and the motion of
the objects in the universe.
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Fgrav ,GRAVITY, is a force which exists
between the Earth and the objects which are near
it.
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As we rise upwards after our jump, the force of
gravity slows us down; and as we fall back to
Earth after reaching our peak, the force of
gravity speeds us up.
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In this sense, the force gravity causes an
acceleration of our bodies during this brief trip
away from the earths surface and back.
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The acceleration of gravity (g) is the acceleration
experienced by an object when the only force
acting upon it is the force of gravity.
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The value of g varies slightly from place to place
on the earths surface.
It is 9.83 m/s2 at the North or South Pole and is
9.87 m/s2 at the equator.
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No matter what their mass, all objects fall
freely at the same acceleration, 9.8 m/s2
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History
Kelplers Three Laws of Planetary System
1. Each planet moves in an elliptical orbit aboutthe sun with the sun at one forces.
(Law of Elliptical Orbit)
2. The line joining the sun and the planet sweepsequal area in equal time ( Law of Area)
3. The ratio of the cube of the mean distance of theplanet from the sun to the square of its period isa constant. (Law of Period)
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Question
If planets do move in the way described byKepler, what keeps them continuously moving in
their orbits.
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The gravitational force exerted by a mass m1 on another
mass m2 is proportional to the products of their masses
and inversely proportional to the square of their
separation r:
F = G m1m2/ r2,
Where G is a constant called the universal gravitation
constant and has a value
G = 6.67 x 10-11 N.m2/kg2
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This means that there are gravitational force
between the sun and the planets, between
the earth and the moon, between the earthand an object near its surface, or between
any two objects.
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Example
The gravitational force between two identical
objects, each with a mass of 10 kg, is 1 N.
What is the distance between their centers?
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WEIGHT
The pull of gravity
Is the gravitational force acting on it by the earth.
Measured in Newtons. If the object falls freely, its weight gives it na
acceleration of g.
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Applying F = ma, the weightW of an object of
mass m is
W= mg
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Objects of greater mass have a greater resistance
to acceleration, but a larger gravitational force
acts on them.
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Another important force that causes centripetal
acceleration is the force of gravity. The quantitative
description of this force was discovered by Isaac Newton.
As he put together the ideas for his three laws of motion,
he also wondered about the circular motion of the planets
about the sun. What prevented these bodies from leaving
their paths?
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According to one of his biographers, Newton was
sitting under an apple tree one day when an applefell.
Newton was struck by the idea that the force
causing the apple to fall might also cause themoon to fall toward the earth.
However, since the moon moved forward as it
fell, the moon followed a circular path ratherthan falling into the earth as the apple had done.
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According to Newton, he began to think of
gravity extending to the orbit of the moon
Using observations about planetary motion byKepler, Newton deducedthat the force that kept
planets in orbit could also account for the earthsgravitational force on an apple.
Newton had developed a logical and basic
understanding of an important aspect of thestructure of our universe.
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1. One pan of the laboratory balance has a mass
of 150 g. The distance between the pans is
250 cm and the force acting on the pans in 12N.What is the mass of the second pan in
grams?
2. Two kids are playing see saw. The masses ofthe kids are 45 kg and 51 kg respectively and
the force acting upon them is 35 N.What is
the distance of separation between the kids?
3. A man is carrying two loads in his hands. Oneof the loads is 4.5 kg while the other is 350 g.
If the distance between his hands is 180 cm,
how much force is he exerting on the loads?