9th Class Unit 5 Notes of Physics

Newton’s law of universal gravitation states that everybody in the universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

The Earth attracts a body with a force equal to its weight.

It is assumed that a gravitational field exists all around the Earth due to the gravitational force of attraction of the Earth.

In the gravitational field of the Earth,

the gravitational force per unit mass is called the gravitational field strength of the Earth. It is 10 N kg-1 near the surface of the Earth.

An object that revolves around a planet is called a satellite.

The moon revolves around the Earth so moon is a natural satellite of the Earth.

Scientists have sent many objects into space. Some of these objects revolve around the Earth. These are called artificial satellites.

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