The weight of a body is the force with which it is attracted towards the centre of earth. When an object is stationary with respect to the planet, its weight equals the gravity. This weight of the object is called as its static or true weight.

We become conscious of our weight, in that case when our weight is opposed by some other object. Usually, the operation of measuring the weight of a body with a weighing machine lies in the fact that as we place the body on the machine, the weighing machine repulses the weight of the object. The reaction of the weighing machine to the body gives the measure of the weight of the body.

The phase of weightlessness may be observed in the following situations:

(i) When objects fall freely under gravity : For example, a lift goes down freely, or an airship showing a feat in which it falls freely for a few seconds in the mean while its flight, are in state of weightlessness.

(ii) When a satellite revolves in its orbit around the earth : Weightlessness poses many serious problems to the astronauts. It seems quite difficult for them to control their positions. Everything in the moon has to be saved tied down. Creation of duplicate gravity is the answer to this problem.

(iii) When bodies are at null points in outer space : On a body shown up, the force of the earth goes on decreasing, but at the similar time the gravitational force of the moon on the body goes on increasing. At one point, the two gravitational forces may be equal and opposite and the net pull on the body converts to zero. This is zero gravity fields or the null point and the body in question is said to appear weightless.

Weightlessness in a Satellite: A satellite, which does not give its own gravity, moves around the earth in a circular orbit under the action of gravity. The acceleration of satellite is GM/r² towards the centre of earth.

If a body of mass m placed on a surface inside a satellite moving around the earth. Then forces on the body are

Thus the field does not exert any force on the body and hence its apparent weight is zero.

A body needs no support to stay at rest in the satellite and hence all position are equally comfortable. Such a state is called weightlessness.

Geo Stationary Satellites:

The satellites which seem at rest relative to earth or which has similar angular velocity as that of earth's rotation i.e. with a time phase of 24 hours, such satellite must be operating in an orbit of specific radius. That orbit is known as parking orbit. If a stationary satellite is at a length h above the earth's field, then its orbiting speed is shown by,             

The time period of its revolution can be given as,  

or  ;  h≈36000km

EFFECT OF GRAVITY:

 Photons and gravity: It is observed that light is affected by gravity. We can understand the gravitational behaviour of light from the fact that although a photon has zero rest mass, it interacts with electrons as though it has inertial mass.

Gravitational Red shift: As we know that an interesting astronomical effect due to the gravitational behaviour of light i.e., the frequency associated with a photon moving towards earth increases and the frequency of a photon moving away from earth should decrease. 

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