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**Hint:**A heavenly body revolving around a planet in an orbit is called a natural satellite. For example, the Moon revolves around the planet earth, so the moon is the satellite of earth and moon is the natural satellite of earth.

**Complete answer:**

The external force for the earth and moon is the gravitational force. Newton’s law of gravitation: It states that everybody with a force which is directly proportional to the product of their masses and is inversely proportional to the square of the distance between them.

\[F\,\,=\,\,G\dfrac{{{m}_{1}}{{m}_{2}}}{{{r}^{2}}}\]

Where, $G$ is the gravitational constant.

Gravitational constant is a scalar quantity. Its value is the same throughout the universe and is independent of the nature and size of the bodies as well as the nature of the medium between the bodies. The value of G in SI unit is \[6.67\times {{10}^{11}}\,N{{m}^{2}}/k{{g}^{2}}\].

**Evidence in support of Newton’s law of gravitation:**

-The rotation of earth around the sun or the rotation of the moon around the earth is explained on the basis of this law.

-The prediction about solar and lunar eclipses, made on the basis of this law, always came out to be true.

-The formation of tides in the ocean is due to the force of attraction between the moon and ocean water.

-The prediction about the orbits and time period of the modern artificial satellites made on the basis of this law are found to be very accurate.

**Important characteristics of Gravitational force:**According to Newton’s law of gravitation, the gravitational force of attraction between two given bodies,

-Is independent of the nature of the intervening medium.

-Is independent of the presence or absence of other bodies.

-Is independent of nature and size of the bodies, till their masses remain the same and the distance between their centre is fixed.

-It forms an action and reaction pair, i.e., the gravitational forces are equal in magnitude and opposite in direction and hence obey Newton’s third law of motion.

-Is a central force as it acts along the line joining the centres of the two bodies.

-It is conservative in nature.

**Hence, the external force for the earth moon system is due to the gravitational attraction.**

**Note:**Newton’s law of gravitation holds good for objects lying at very large distances and also at very short distances. It fails when the distance between the object is less than \[{{10}^{-9}}\] m (i.e., the order of intermolecular distance) because in this range repulsive force is dominant over attractive force.

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