Class 9 Science

Chapters

# Gravitation

• Universal Law of Gravitation
• SI unit of G

Earth attracts everything towards it by an unseen force of attraction. This force of attraction is known as gravitation or gravitation pull.

In fact, every object in the universe attracts all other objects by this force. In other words, every object in the universe has gravitation or gravitation pull. The sun, the moon, planets and stars, even a rubber has gravitation or gravitation pull.

## Universal Law of Gravitation

Every object in the universe attracts other object by a force of attraction, called gravitation, which is directly proportional to the product of masses of the objects and inversely proportional to the square of distance between them. This is called Law of Gravitation or Universal Law of Gravitation.

The distance is considered between the centres of the objects.

Suppose there are two objects having mass M and m respectively.

The distance between their centres is equal to d.

The force of attraction is F.

Therefore, from Law of Gravitation which states that force of attraction by which an object attracts other object is directly proportional to the product of their masses,

Thus, FpropM*m -----(i)

Now, Law of Gravitation also states that force of attraction by which an object attracts other object is inversely proportional to the square of distance between them.

Therefore, F prop 1/d^2 ----(ii)

Now from equation (i) and (ii) we get

F prop (M*m)/d^2
=>F=G*(M*m)/d^2 -----(iii)

Where, G is the proportionality constant and called Universal Gravitation Constant.

From equation (iii)

Fxxd^2 = G*M*m
=>G= (Fd^2)/(M*m) ----(iv)

The expression (iii) and (iv) are called expression for Universal Law of Gravitation.

This Law is applicable everywhere in universe, thus it is known as UNIVERSAL LAW OF GRAVIATION.

## SI Unit of G

The SI unit of G is Nm^2kg^(-2)

The accepted value of G is 6.673xx10^(-11)Nm^2\ kg^(-2)

The value of G was found out by Henry Cavendish, a British philosopher and scientist.

#### Importance of Universal Law of Gravitation

• This Law is applicable to every object in the universe.
• This law explains the cause of revolution of moon around earth and revolution of planets around sun.
• This law explains the cause of neap and tide due to moon and sun.

## Quiz

1: (b) Product of their masses, 2: (d) Square of distance between bodies, 3: (a) Nm2Kg-2, 4: (c) Gravitational force only, 5: (b) It is more at poles than at equator, 6: (a) 9.8 m s-2, 7: (d) Acceleration, 8: (c) Mass and g, 9: (b) Kilogram, 10: (d) Newton

## Matter in Our Surroundings

Anything that has both mass and volume is called matter. You can also say that anything which has mass and which occupies space is called matter.

## Is Matter Around Us Pure?

Elements and compounds are pure substances. All other substances are mixtures which means they are not pure substances.

## Atoms and Molecules

Read about law of conservation of mass, law of constant proportions and Dalton's atomic theory.

## Structure of Atoms

Atom is made of three particles; electron, proton and neutron. These particles are called fundamental particles of an atom or sub atomic particles.

## Cell: The Fundamental Unit of Life

A cell is capable of independent existence and can carry out all the functions which are necessary for a living being.

## Tissue

A groups of cells which is meant for a specific task is called tissue. Tissues are the first step towards division of labour in complex organisms.

## Diversity in Living Organisms

Without proper classification, it would be impossible to study millions of organisms which exist on this earth.

## Motion

If an object changes its position with respect to a reference point with elapse of time, the object is said to be in motion.

## Force & Laws of Motion

Force has numerous effects. Force can set a stationary body in motion. Force can stop a moving body.

## Gravitation

Earth attracts everything towards it by an unseen force of attraction. This force of attraction is known as gravitation or gravitation pull.

## Work & Energy

When force is exerted on an object and object is displaced, work is said to be done. It means work is the product of force and displacement.

## Sound

Sound is a type of energy. Sound travels in the form of wave from one place to another.

## Why Do We Fall Ill

Health is a state of physical, mental and social well being. A condition in which the affected person is unable to carry out normal activities is termed as disease.

## Natural Resources

Resources which are obtained from nature are called natural resources. Examples: Air, water, soil, wood, etc.

## Improvement in Food Production

Food security is said to exist when all people, at all times, have physical and economic access to sufficient, safe and nutritious food to meet their dietary needs and food preferences for an active and healthy life.