Welcome students
I presume you have completed your rotational dynamics along with videos and certain extra assignments which are given on the website. Now we talk about the next topic, and that is gravitational field. This is one of the easiest topics in Physics and having the same concept as electrostatics. Since you have already done electrostatics recently it will be very easy for you to understand the basic formulas and the concept of gravitational field. Force between two charges and the force between two masses is having the same formula excepting, the mass is replaced by charge. Similarly the electrical field intensity and the gravitational field intensity are to have the same formula accepting the constants. Electrostatics formulas have electrical constant and in gravitational formulas we have gravitational constant.
So here we start Gravitational field and Gravitation chapter. First thing to understand is between two masses, if one of the mass is taken to be mass of Earth and other as 1 kg then the formula becomes the acceleration due to gravity, whose value is 9.8 m/s2 on the surface of e
The formula for acceleration due to gravity, does change with height above the surface and depth. At the height we have got two formulas, first is when the height is more than 5 percentage of radius that is roughly more than 500 kilometers, while second formula is to be used when the height is less than 500 kilometers. So far as the value of gravity depending on the depth is concerned, we have two very basic assumptions, first is that Earth is a perfect sphere and second one is that it has got uniform density. Both concepts are not hundred percent correct but still we assume it to be correct enough for calculation purpose.
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</script>Next we discuss about satellite. We have got natural as well as man made satellite. Moon is satellite of Earth and is natural, hence said to be a natural satellite. Earth can be said to be a satellite of Sun. Natural satellites have different time periods but usually the satellite which are man made comma are used for some communication purposes. Hence man made satellites are usually having geostationary property. These geostationary satellites, remain stationary as compared to earth. This geostationary satellites has the time period 24 hours. They have same sense of rotation same as that of Earth.
The orbital velocity is an important aspect in satellite. The orbital velocity produces the necessary centripetal force which is balanced by the gravitational pull between satellite and Planet.
Here we come across three laws of John Kepler.
First law of orbits states that every planet moves around Sun in an elliptical Orbit which sun at one of its foci.
Second law states that areal velocity, the velocity of area swept is will be same in equal intervals of time because of this only when the planet is closer to sun it will move faster when it is farther apart it will be less. Next we talk about, what is called the law of time period, it states square of time period is directly proportional to the cube of orbital radius.
Lastly we talk about the escape velocity and gravitational potential. Gravitational potential is defined as the work done in bringing unit mass from infinity to that point against gravitational force. It is having almost, the same definition as of electrical potential but the field is electrical in that. The concept of escape velocity gives the minimum velocity which, given to a particular body near the Earth’s surface will escape the earth’s gravitational pull. It is obtained by equating potential energy of the body.
Now if velocity is less than escape velocity body will come back after a certain height , which can be obtained by equating potential energy at that point and kinetic energy to be zero. If we have exactly same as escape velocity then the body will leave Earth’s gravitational pull. And lastly if velocity is given more than the escape velocity body will not only leave gravitational pull, but will have kinetic energy in due course as well. It is explained clearly in the video.