Time is nature's way of keeping everything from happening at once.
~Woody Allen
Time appears in physics is as another component in the address of an event.It's hard to know how to do that people talk about time, flowing a common question that gets asked of such people is how fast is it flowing? The normal answer is: I don't know. [........] The next question is: What would it be like if it were flowing two seconds per second instead of one second per second?
In nature, we see that if something moves, it changes its location. It takes some time to complete that movement. So the change in location over a time is defined as speed (or, its rate of change). If the thing is moving in a particular direction then the speed is defined as velocity.
Mathematically,
Velocity (v) = $\frac{distance (x)}{time (t)} $
Velocity is the rate (or speed) an object is moving from A to B over a measurable time.
It's not possible to maintain a constant speed for a very long time. At some point, the speed will increase (or, decrease) or change the direction of motion. All of these changes take place over a time, which are in the form of acceleration.
Acceleration is the rate (or speed) at which an object is increasing or decreasing its velocity over a measurable time.
For example: Firstly, we notice that the object change its original position with the certain motion. In the first second, it moved at a speed of 1 meter per second. But after ten seconds, the object travels 2 meters per second. So in 10 seconds, it's speed has increased by 1 meter per second. This is called acceleration.
Finally, let's try to explain this concept in a scientific point of view:
Acceleration as doing two things at once. "We are still moving across a distance over a time, but we are also increasing how fast we are doing it." We are multi-tasking to arrive sooner. So, time is repeated twice in the unit of acceleration.
Hence, time appears twice: once to describe the rate at which position is changing (i.e. the speed) and once to describe the rate at which the speed is changing.
The Natural force that tends to cause physical things to move towards each other.
First, [Newton's Law of Universal Gravitation] is a mathematical in its expression.... Second, it is not exact. Einstein had to modify it.... There is always an edge of mystery, always a place where we have some fiddling around to do yet.... But the most impressive fact is that gravity is simple.... It is simple, and therefore it is beautiful.... Finally, comes the universality of the gravitational law and the fact that it extends over such enormous distances....
~ Richard P. Feynman.
Sir Issac Newton:Albert Einstein:
$F = G\frac{m_1 m_2}{r^2}$
$G\mu \nu = 8\pi GT\mu \nu$
» It is a force.
» It is a distortion of space and time.
» It depends on mass and distance.
» It depends on Energy.
» Newton's stated that the force of gravity is always attractive. It affects everything with mass, work instantaneously at a distance and has an infinite range. "Every mass attracts every other mass in the universe (across the empty space)", Newton had imagined. [1]
» But in Einstein's model, gravity is not a force. It is a warping of space-time. Space is really curved, and as a result objects are deflected from a straight path in a way that looks like a force. According to Einstein: "Mass tells space how to bend, and space tells mass how to move".
