Notes On Earth Satellites and Weightlessness - CBSE Class 11 Physics
Early in our lives, we become aware of the tendency of all material objects to be attracted towards the earth. Everything thrown up falls downwards the earth, going uphill is lot more tiring comparatively than going downhill, raindrops from the clouds fall down towards the earth and there are several other such phenomena. Historically Galileo (the Italian Physicist) recognised the fact that all bodies, irrespective of their masses, are accelerated towards the earth with a constant acceleration   Copernicus proposed Heleo centric theory which states that the planets move around the sun in circles as sun is at the centre of the circle. His theory was discredited by the church, but notable amongst its supporters was Galileo who had to face prosecution from the state for his beliefs.   It was around the same time as Galileo, a nobleman namely Tycho Brahe of Denmark, recorded the observations of the planets with the naked eye throughout his entire life. His compiled data were analysed later by his assistant Johannes Kepler.   Kepler extracted three elegant laws from the data recorded by Tycho Brahe. These laws are now go by the name of Kepler’s laws. These laws were known to Newton and enabled him to make a great scientific leap in proposing his Newton’s universal law of gravitation.   Satellites of the earth are objects which revolve around the earth. The motion of the satellite of the earth is very similar to the motion of planets around the Sun and hence Kepler’s laws of planetary motion are equally applicable to them.    In particular, their orbits around the earth are circular or elliptic. Moon is the only natural satellite of the earth with a near circular orbit with a time period of 27.3 days approximately which is almost equal to the rotational period of the moon about its own axis.   There are two types of artificial satellites. They are geo-synchronous and Sun-synchronous satellites.   In geo-synchronous satellites, motion in their orbit is synchronised with the rotation of the earth.   Geo-synchronous satellites are used for telecommunications and mass communications. In sun-synchronous satellites, the motion of the satellite is synchronised with the local sun time at a given place.   Sun-synchronous satellites have various applications such as remote sensing, environmental studies, weather analysis and forecasting.   A body experiences zero-gravity or weightlessness in a free fall. Cosmonauts in manned satellites experience weightlessness.   Geo-stationary - polar satellites and weightlessness. Communication plays a vital role in today’s world. Satellites are used for effective communication. Apart from communication satellites are used for various purposes ranging from analysis of weather conditions in a region. Live transmission of cricket matches through television, exploration of minerals from the earth and for geographical purposes; satellites used for various purposes are artificial satellites.   Indian Remote Sensing satellites, generally referred to as IRS which are used for remote sensing applications, environmental studies, weather analysis, forecasting and similar applications are examples of polar satellites.   Artificial satellites are generally unmanned. At times, cosmonauts are sent along with satellites into space to conduct certain studies. In space, they feel zero-gravity or weightlessness.   The weight of a body is the force experienced by the body due the gravitational pull of the earth.   A spring balance that measures the weight of a body works on the principle of extension of the spring under the action of the weight of the body attached to the balance, which is fixed to a rigid support. While measuring the weight of the body attached to the balance, the body is pulled downwards due to the earth’s gravity. It, in turn, exerts the same force on the spring downwards, causing its extension. The spring, in turn, pulls the body upwards with the same force and this is indicated in the reading of the balance. This is the normal force exerted by the spring on the body. If the balance is not suspended to the rigid support and is allowed to fall freely, then both the body and the spring balance fall down with acceleration due to gravity. In this case, the normal force exerted on the body by the balance is zero; hence the stretching of the spring is zero. Thus, the spring balance reads zero weight of the body. This situation is the weightlessness of the body. Thus we can say that a body experiences zero-gravity or weightlessness in a free fall.   In the case of a satellite orbiting the earth, it is similar to any other freely falling body, but it has very high velocity in a tangential direction of its orbit due to which it does miss the earth as it falls. Thus, every part of the satellite is in a state of free fall and it is subjected to acceleration due to gravity in that position. Hence, cosmonauts in manned satellites experience weightlessness.

#### Summary

Early in our lives, we become aware of the tendency of all material objects to be attracted towards the earth. Everything thrown up falls downwards the earth, going uphill is lot more tiring comparatively than going downhill, raindrops from the clouds fall down towards the earth and there are several other such phenomena. Historically Galileo (the Italian Physicist) recognised the fact that all bodies, irrespective of their masses, are accelerated towards the earth with a constant acceleration   Copernicus proposed Heleo centric theory which states that the planets move around the sun in circles as sun is at the centre of the circle. His theory was discredited by the church, but notable amongst its supporters was Galileo who had to face prosecution from the state for his beliefs.   It was around the same time as Galileo, a nobleman namely Tycho Brahe of Denmark, recorded the observations of the planets with the naked eye throughout his entire life. His compiled data were analysed later by his assistant Johannes Kepler.   Kepler extracted three elegant laws from the data recorded by Tycho Brahe. These laws are now go by the name of Kepler’s laws. These laws were known to Newton and enabled him to make a great scientific leap in proposing his Newton’s universal law of gravitation.   Satellites of the earth are objects which revolve around the earth. The motion of the satellite of the earth is very similar to the motion of planets around the Sun and hence Kepler’s laws of planetary motion are equally applicable to them.    In particular, their orbits around the earth are circular or elliptic. Moon is the only natural satellite of the earth with a near circular orbit with a time period of 27.3 days approximately which is almost equal to the rotational period of the moon about its own axis.   There are two types of artificial satellites. They are geo-synchronous and Sun-synchronous satellites.   In geo-synchronous satellites, motion in their orbit is synchronised with the rotation of the earth.   Geo-synchronous satellites are used for telecommunications and mass communications. In sun-synchronous satellites, the motion of the satellite is synchronised with the local sun time at a given place.   Sun-synchronous satellites have various applications such as remote sensing, environmental studies, weather analysis and forecasting.   A body experiences zero-gravity or weightlessness in a free fall. Cosmonauts in manned satellites experience weightlessness.   Geo-stationary - polar satellites and weightlessness. Communication plays a vital role in today’s world. Satellites are used for effective communication. Apart from communication satellites are used for various purposes ranging from analysis of weather conditions in a region. Live transmission of cricket matches through television, exploration of minerals from the earth and for geographical purposes; satellites used for various purposes are artificial satellites.   Indian Remote Sensing satellites, generally referred to as IRS which are used for remote sensing applications, environmental studies, weather analysis, forecasting and similar applications are examples of polar satellites.   Artificial satellites are generally unmanned. At times, cosmonauts are sent along with satellites into space to conduct certain studies. In space, they feel zero-gravity or weightlessness.   The weight of a body is the force experienced by the body due the gravitational pull of the earth.   A spring balance that measures the weight of a body works on the principle of extension of the spring under the action of the weight of the body attached to the balance, which is fixed to a rigid support. While measuring the weight of the body attached to the balance, the body is pulled downwards due to the earth’s gravity. It, in turn, exerts the same force on the spring downwards, causing its extension. The spring, in turn, pulls the body upwards with the same force and this is indicated in the reading of the balance. This is the normal force exerted by the spring on the body. If the balance is not suspended to the rigid support and is allowed to fall freely, then both the body and the spring balance fall down with acceleration due to gravity. In this case, the normal force exerted on the body by the balance is zero; hence the stretching of the spring is zero. Thus, the spring balance reads zero weight of the body. This situation is the weightlessness of the body. Thus we can say that a body experiences zero-gravity or weightlessness in a free fall.   In the case of a satellite orbiting the earth, it is similar to any other freely falling body, but it has very high velocity in a tangential direction of its orbit due to which it does miss the earth as it falls. Thus, every part of the satellite is in a state of free fall and it is subjected to acceleration due to gravity in that position. Hence, cosmonauts in manned satellites experience weightlessness.

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