law of gravitation

8/4/2019 Law of Gravitation

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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?

law of gravitation

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