# Class 9 Science notes Gravitation

Centripetal force, Gravitation, NEWTON’S UNIVERSAL LAW OF GRAVITATION, IMPORTANCE OF THE UNIVERSAL LAW OF GRAVITATION, THRUST AND PRESSURE, PRACTICAL APPLICATIONS OF ARCHIMEDES PRINCIPLE and much more …… Class 9 Science notes Gravitation

## Centripetal force

The external force F that causes acceleration and keeps the body moving uniformly along the circular path is acting towards the centre of the circular path. This is called centripetal force.

• Motion of the Moon around the Earth is due to centripetal force, provided by force of attraction of the Earth on the Moon. Similarly, as all the planets of our solar system revolve around the sun.

## GRAVITATION

Gravitation as the phenomenon of attraction between any two objects in the universe. The objects may be terrestrial (which are on the Earth) or celestial (which are in outer space) such as stars, planets, satellites etc. Further, the objects may be of any size, shape or mass and they may be any distance apart (small or big), with any medium between them.

## NEWTON’S UNIVERSAL LAW OF GRAVITATION

Every object in the Universe attracts every other object with a force which is:

1. directly proportional to the product of their masses, and
2. Inversely proportional to the square of the distance between their centres.

F = GM1M2/R2

The direction of the force is along the line joining the centres of two objects.

Universal gravitational constant G is numerically equal to the gravitational force of attraction between two bodies, each of unit mass (one kg) kept at unit distance (1 metre) from each other.

### IMPORTANCE OF THE UNIVERSAL LAW OF GRAVITATION

1. The gravitational force of attraction of the Earth is responsible for binding all terrestrial objects on the earth.
2. The gravitational force of the Earth is also responsible for the rainfall and snowfall on the Earth.
3. The moon revolves around the Earth on account of gravitational pull of the Earth on the Moon.

Even all artificial satellites revolve around the Earth on account of gravitational pull of the Earth on the satellites.

## KEPLER’S LAWS OF PLANETARY MOTION

1. Kepler’s First Law

Every planet revolves around the Sun in an elliptical orbit, with the sun situated at any one  of the foci (plural of focus) of the ellipse.

• Kepler’s Second Law.

In the elliptical orbit of the planet, the line joining the centre of the planet to the centre of the Sun sweeps equal areas in equal intervals of time.

• Kepler’s Third Law.

The square of time period of revolution of a planet around the Sun is directly proportional to the cube of the semi major axis of the elliptical orbit.

It is important to note that Kepler’s stated only the laws of planetary motion, but he could not give a theory to explain the motion of planets. It was Newton who established that the real cause of the planetary motion is the gravitational force that is exerted by the Sun on the planets.

HOW DID NEWTON GUESS THE INVERSE SQUARE RULE?

The gravitational force F between any two bodies is inversely proportional to the square of the distance (r) between them.

GRAVITY

Gravity is the phenomenon of attraction between Earth and any other body.

## FREE FALL

All objects falling towards earth under the action of gravitational force of earth alone are said to be in free fall.

• The value of g does not depend upon mass of the body. But gravitational pull of Earth F =mg, depends on mass of the body. Heavier the body, greater is the gravitational pull of Earth on it and vice-versa.

MASS

The mass of a body is defined as the quantity of matter (or material) contained in the body. It is represented by m.

WEIGHT

The weight of a body is defined as the force with which the body is attracted towards the centre of the Earth. In other words, weight of a body is the force of Earth’s gravity acting on the body.

WEIGHT OF AN OBJECT ON THE MOON

The weight of an object on Moon is the force with which the object is attracted towards the centre of the Moon. In other words, weight of an object on Moon is the force of moon’s gravity acting on the object.

## THRUST AND PRESSURE

Force acting normally on a surface is called the thrust.

• A sharp knife cuts easily than a blunt knife by applying the same force.
• A sharp needle pressed against our skin pierces it. But a blunt object with a wider contact area does not affect the skin when pressed against it with the same force.
• Smaller the area on which the force acts, greater is the impact.
• The thrust acting on unit area of the surface is called the pressure.

Some interesting aspects of pressure

1. The foundation of a building or a dam has a large surface area so that the pressure exerted by it on the ground is less. This is done to prevent the sinking of the building or the dam into the ground.
2. Nails and pins have pointed ends so that these can be fixed with minimum force because the pressure on the pointed ends would be large.
3. Wide wooden sleepers are kept below railway lines to reduce pressure on the railway tracks and prevent them from sinking into ground.

DENSITY

Density of a substance is defined as its mass per unit volume.

## RELATIVE DENSITY

Relative density of a substance is defined as the ratio of its density to that of water at 4.C.

Relative density can also be defined as the ratio between the mass of the substance and the mass of an equal volume of water at 4.C.

PRESSURE IN FLUIDS

In an enclosed fluid, if pressure is changed in any part of the fluid, then this change of pressure is transmitted undiminished to all the other parts of the fluid.

## BUOYANCY

When a body is partially or wholly immersed in a liquid, an upward force acts on it which is called up thrust or buoyant force. The property of the liquids responsible for this force is called buoyancy.

The up thrust on a body due to a liquid arises on account of pressure difference between the lower and the upper parts of the body immersed in the liquid.

If the density of the material of the body is less than that of density of water, it floats and in case if the density of the material of the body is more than that of water, the body sinks.

A body having an average density greater than that of water (liquid, in general), sinks into it while a body of average density smaller than that of water (liquid, in general), floats on it.

## ARCHIMEDES PRINCIPLE

When a body is immersed partially or wholly in a liquid at rest, it experiences an up thrust which is equal to the weight of the liquid displaced. The apparent loss in weight of the body is equal to the up thrust on the body.

### PRACTICAL APPLICATIONS OF ARCHIMEDES PRINCIPLE

1. Designing ships and submarines
2. Determining the purity of a sample of milk by using an instrument, called lactometer.
3. Determining the density of liquids by using an instrument, called hydrometer.
4. Checking the concentration (density) of sulphuric acid in acid batteries by using an instrument, called acid battery hydrometer.

Pascal’s law states that the pressure applied at any place to an enclosed mass of a fluid is transmitted equally in all directions and it acts undiminished at every point of the fluid and on the walls of the container.