# NCERT Class 9 Physics Formula and Important Terms Board Sample Problems Part 3 (For CBSE, ICSE, IAS, NET, NRA 2022)

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## Gravitation

What is Gravitation?

Gravitation is the force of attraction between the any two bodies having mass

 S. no Points 1 Laws of nature are same for earthly and celestial Bodies. 2 Kepler (1571 - 1631) Studied the planetary motion in detail and formulated his three laws of planetary motion, which were available Universal law of gravitation. 3 Gravitational force or gravity of earth is responsible for pulling you and keeping you on earth. 4 Fact that all bodies irrespective of their masses are accelerated towards the earth with a constant acceleration was first recognized by Galileo (1564 - 1642) .

### Universal Law of Gravitation

As stated by Newton՚s

Everybody in 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 distance between them. The force acts along the line joining the two bodies

Here G is the constant known as Universal Gravitational constant.

SI unit of gravitational constant G is

Value of G was first found by Henry Cavendish (1731 - 1810) and it is

 S. no Points 1 The gravitational force is a central force that is It acts along the line joining the centers of two bodies. 2 It is a conservative force. This means that the work done by the gravitational force in displacing a body from one point to another is only dependent on the initial and final positions of the body and is independent of the path followed. 3 It is always attractive in nature 4 It is responsible for the motion of planets around the sun, motion of moon around earth 5 The tides formed by rising and falling of water level in the ocean are due to the gravitational force exerted by both sun and moon on the earth 6 Artificial satellites revolve around the earth.

### Free Fall

The falling of a body (or object) from a height towards the earth under the gravitational force of earth (with on other force acting on it) is called free fall whenever the objects fall towards earth an acceleration is involved due to earth՚s gravitational force.

This acceleration due to earth՚s gravitational force is called acceleration due to gravity which is denoted by ‘g’ and its unit is m/ g value on earth surface is around 9.8 m/

### Motion under Free Fall

when object is falling downwards

when object is thrown upwards

and

Where h is the height of the object from the ground.

### Difference between Acceleration Due to Gravity (G) and Universal Gravitational Constant (G)

 Acceleration due to gravity (g) Universal gravitational constant (G) It is the acceleration produced in a freely falling object under the action of the earth՚s Gravitational force. It is the gravitational force of attraction between two objects of unit masses separated by a unit distance The value of g is different at different places on the earth as well as other planets. The value of G remains same everywhere in the universe.

### Interesting Facts About Force and Gravitation

 S. no Points 1 Unlike the Force, with its dark and light sides, gravity has no dual nature; it only attracts, never repels. 2 Gravity is by far the weakest of the four fundamental forces. The other three are electromagnetism; weak nuclear force, which governs how atoms decay; and strong nuclear force, which holds atomic nuclei together. 3 Gravity takes time, meaning that if the sun disappeared, Earth would still orbit it for as long as we saw light from it (about 8 minutes) 4 The gravitational force on the surface of Earth accelerates every object at the same rate, regardless of its weight. If you drop a big box and a small box in vacuum from a certain height, they would hit the ground at the same time. 5 Einstein gave a whole new concept of gravity. His general theory of relativity was the first to treat gravity as a space-time distortion. He described space-time as a “fabric” that physically embodies the universe. Anything with a mass wraps the space-time around it. 6 A force is a push or a pull. It can make something start to move, slow down or speed up, change direction or change shape or size. 7 Mass is the measure of inertia. Or in other words Inertia is measure to mass of an object. The larger is the mass, larger is the inertia and vice-versa

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