As the physics are differentiated into 3 groups:
1) Classical Physics
2)Relativity
3) Quantum Physics
Among them quantum physics don't deal with gravity much except the string theory.
Classical physics and relativity however deals with gravity in many cases.
In classical physics the definition of gravity was given by Sir Issac Newton. He defined that gravity is the force of attraction of a matter that draws the matter with comparatively less mass towards its center.
This definition can be little doubtful in one case.
We know that the Galileo Galilei conducted an experiment in which he proved that what ever the mass of matter is, all matter fall at the same rate of acceleration towards the earth where the air resistance is negligible.
There he threw a a large mass and a small mass from leaning tower of pisa and from that he noticed that both mass reach on the ground at the same time.
In Newtonian Physics or the Classical Physics, this experiment is show by this process:
The force acting on both free falling body is
Where
M= the earth's mass
G= Gravitational constant
r= the distance between the center of the freefalling object and the center of the earth
m1 on the LHS= gravitational mass of object 1
m1 on the RHS = inertial mass of the object 1
Similarly m2 on the LHS is gravitational mass of object 2 and m2 on the RHS is the inertial mass of object 2
We got new term like gravitational mass and inertial mass.
Gravitational mass - the property of an object that creates and experiences the gravitational field
Inertial mass- is the measure of resistance to acceleration.
But why those masses are identical to each other. Why they are cancelling each other?
Why RHS m1 and LHS m1 cancel out despite they have different meaning.
Regarding this many scientist performed experiments to proved that both mass are identical.
But in theory of relativity there is no such doubt
According to Einstein theory of relativity gravity is not a force. Gravity is not the object pulling towards each other. Imagine that you are in a elevator. Suddenly it freefalls what do you feel? It feels that you are weightless there is no such gravity all the object in your frame of reference levitate. With you frame of reference you feel that you are in a deep space or something like you are the astronaut in ISS. But with the others frame of reference it clearly seems that you are falling. The physics experience depends upon the experience of person so you who is falling in your frame of reference it feels that you are in a deep space.
Now imagine it's vice-versa. When the lift is accerating upwards. You on the lift feel more weight. Its feels similar to gravity. Now imagine the same situation on the ISS . You were levitating just before few minutes now suddenly the engines shuts on and the iss move. Here the force on the rocket pushes it up and the the one you inside gets down and similar thing happens, you feel weighty. Now here without any gravitational field you have experienced gravity. This is the thing relativity trying to say. We on the earth are accerating upward. None of the force is pulling towards the earth center. But if we are accerating upward then why we don't experience. It because it need an inertial frame of reference. Just like you freefalling on the elevator feels no weight but the other out side the elevator they are the inertial viewers that clearly see that you are falling. Similarly, you on the elevator who is feeling weight observes that the others outside the elevator accerating upwards. So according to your frame of reference or the inertial frame of reference, it clearly see that the matter on the earth surface accerating upwards.
Hence , according to the relativity theory, there is only one force that is accerating all the matter upwards and giving experience of gravity. It is only observed by a inertial frame of reference that all the matter on the earth are accerating upward.
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