Saturday, April 24, 2010

Earth's Satellites

A heavenly body that revolves round the earth in a circular orbit is called earth's satellite.For example,the moon is the earth's satellite which revolves round the earth in a circular orbit of radius 3.84*10^5km.The moon takes about 27.3 days to move once round the earth.The earth's satllite are of two types: natural satellites and artificial satellites.

Natural satelite:
It is a naturally formed body moving around the earth.The moon is the only natural satellite of earth.

Artificial satellite:
It is man made body moving around the earth in an orbit.When a body is taken to a certain height above the earth and given necessary centripetal force, the body keeps on moving around the earth and becomes an atrificial satellite.

Nowadays, the artificial satellites find widenespread applications in the present worrld.The following are the uses of artificial satellites:
  • Artificial satellites are widely used in telecommunication.
  • They are used in studying the atmosphere near the earth surface.
  • They are used to forecast weather.
  • They are used to transmit rafio and video signals.
  • They are used to know the shape and size of the earth.
  • They are used in space flights.
  • They are used to study the radiations coming from the sun and the outer space.

Thursday, April 22, 2010


Weightlessness is the state of a body at which he weight of a body is zero.During freefall, the reaction of force may be zero.So weight, also defined as measure of reaction force, becomes zero at that state.Also outside the gravitational field the body can have zero weight.These are simply called weightlessness.

Weightlessness can be classified into two types:-
  • True weightlessness
  • Apparent weightlessness

True weightlessness:

The state of body at which the weight of a body is zero due to absense of acceleration due to gravity.Outside gravitational field, mass(m) can never b zero but acceleration due to gravity(g) is zero.And we know weight(W)=mg,


.;. W=0

Apparent weightlessness:

Simply the weight of a body while its in freefall is called apparent weightlessness.This is due to lack of reactant force to the body.

Differences between Mass and Weight


  • It is scalar quantity.
  • It is a measure of inertia in a body.
  • It's value remain constant at all places.
  • It can never be zero.
  • It is measured by a beam balance.
  • It's unit is kilogram.


  • It is vector quantity.
  • It is a measure of the force of attraction of the earth on the body.
  • It's value varies from place to place.
  • It is zero at places where g=0.
  • It is measured by spring balance.
  • It's unit is Newton

Differences between inertial Mass and Gravitational Mass

Inertial mass

  • Inertiam mass is measure of opposition to acceleration.
  • It is measured by Newtion's II law of motion.
  • The inertial mass can be measured only when the body is moving.

Gravitational mass

  • Gravitational mass is measure of force of attraction between the body and the earth.
  • It is measured by Newton's law of gravitation.
  • Gravitational mass can be measured when the body is at rest i.e., by using common balance.

Differences between Gravity and Gravitation

Usually gravity and gravitation are used for same thin n mostly as synonyms.But literally there is vast differences between them.

  • Gravitation is the force of attraction between two bodies with masses as described as Newton's first law .
  • Gravity is the special case of Gravitation in which one mass is of earth or any other planet.
  • Since Gravitation is applied in any galaxy,atom or body it is universal.So called universal gravitaional constant.

Fore more see here

Saturday, April 17, 2010

Mass and Weight

Inetria of mass
The mass of the body defined by Netwon's Seond law of motion is called inertial mass.From
Newton's 2nd law, if a force F applied on the body produces acceleration a on it, then, we have F=ma.

here quantity m is called the inertial mass of the body.Thus, the inertial mass (m)=F

Suppose the same force is applied on two bodies. The body in which the accleration produced is less has more inertial mass than that of teh body in which the accn is produced is more.

Thus, the inertial mass of a body measures of its inertia.It is called as force applied per unit acceleration produced.

Friday, April 16, 2010

Acceleration due to gravity

The Universal Gravitational Constant (G)

(G)The value of G for any pair of masses in the universe is the same.On the other hand its value is independent of the medium in which the masses are placed.This is why the G is called universal gravitational constant.


If m1 and m2 are unit masses and separated by unit distance, then F=G.

Hence, the universal gravitational constant can be defined as the gravitational force between two unit masses which are separated by a unit distance.

Sunday, April 11, 2010

Explanation of Netwons Law of Gravitation

According to Newton;s Law of Gravitation,

  • F directly proportional to m1m2 .......(i)
  • F directly proportional to 1/r^2 .......(ii)

Combining both equation,

F=G(m1m2)/r^2 where G is universal gravitational constant.[G=6.67*10^-11]

Newton's Universal Law of Gravitation

Newton was able to formulate the universal law of gravitation when he obserbed the motion of the planets around sun.

Statement:Newton's universal law of gravitation states that everybody in this universe attracts other body with a force which is:
  • directly proptional to the product of their masses and
  • inversely proptional to the sruare of distances between them.The direction of force is along the line joining the bodies.

Gravitational force

Newton,in 1687, discovered the existence of force between two masses.For example, two people sitting in room,a table and a rock lying in a room,sun and the earth.
The force between two masses is called GRAVITATION FORCE.The gravitational force is attractive.this is each body attracts towards it the other body.this force exists universally among all bodies in universe.

In gravitational force,we talk about the force of attraction between two bodies, out of these bodies, if one body is the earth, the gravitational force is called force of gravity.thus, the force of gravity is special case of the force of gravitation.

When a stone is released from the top of a tower, it falls towards the ground. Similarly, if a vody is thrown upward, its speed slows down.The speed becomes zero when the body reaches certain height.After the maximim height, the body begins falling down.Finally, the body returns to the ground.It implies that the earth attracts every body lying near its surface and on its surface towards its centre.The force of attraction of earth on the bodies lie on its surface is called gravity or force of gravity.

Not only earht but all the planets or heavenly bodies have gravity.the gravity of a body depends upon its mass.The gravity of moon is about 1\6th times the gravity of earth.