# Physics : Gravitation -1

**Gravitation**:- Gravity, or gravitation, is a natural phenomenon by which all things with mass are brought toward (or gravitate toward) one another, including objects ranging from electrons and atoms, to planets, stars, and galaxies.

**Newton’s law of Gravitation:-**

- Gravitational force is an attractive force between two masses and . Separated by a distance ‘r’ “

- The gravitational force acting between two point objects is proportional to the product of their masses and inversely proportional to the square of the distance between them.

Where,

- F1 and F2 are gravitational forces between the two bodies.
- G is the gravitational constant
- m1 = mass of first body
- m2 = mass of first second body
- ‘r’ is the distance between the two bodies

**Gravity (G) :- **

- It is the gravitational force of earth
- Gravity is the force by which earth pulls a body

**According to gravity (g):**

The acceleration produced in a body due to gravity is called acceleration due to gravity.

- Denoted by ‘g’ and its value is 9.8 m/s2

__Important facts about ‘g’__

Value of ‘g’ decreases with height or depth from earth’s surface

- ‘g’ is maximum at poles.
- ‘g’ is minimum at equator
- ‘g’ decreases due to rotation of earth
- ‘g’ decreases if angular speed of earth
- ‘g’ decreases due to rotation of earth increases and increases if angular speed of earth decreases.

Angular speed:- It is “the rate at which an object changes its angle, in a given time period”

__Weight of a body in a lift.__

- If lift is stationary or moving with uniform speed.

Apparent weight = True weight

- If lift is going up with acceleration

Apparent weight > True weight

- If lift is going down with acceleration
- If the cord of the lift is broken it falls freely in this situation the weight of a body in the lift becomes zero.

**Weightlessness: –** It is the situation in which the effective weight of the body becomes zero.

**Important question asked in SSC and other exams –**

**Q.1 Presence of atmospheric air on the earth is due to –**

(a) Gravity

(b) By wind

(c) Clouds

(d) Rotation of earth.

**Ans. (a) ** The gravity of our earth holds the atmospheric in its place

**Q.2 In the earth, the weight of a body maximum at the –**

(a) Equator (b) Centre (c) Surface (d) poles

**Ans. (d) ** Gravity is maximum at the poles. Hence. Weight of any body will be maximum at poles.

The weight of an object is minimum at the center of the earth.

**Q.3 Law of gravitation applies to –**

(a) Any pairs of bodies

(b) The earth and the moon

(c) The Planets around the sun

(c) The earth and the planet around the sun

(d) The earth and the objects of earth

**Ans. (a)** Newton’s law of gravitation applies to any pair of the bodies in the universe.

**Q.4 What is the value of acceleration due to gravity at the centre of earth?**

(a) -1 (b) 1 (c) 0 (d) Infinite

**Ans. (c)** At the centre of the earth, Gravity will be zero because there is equal mass pulling on a body from all side and it all gets cancel.

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