what is gravity???地心引力 ?? english

Gravity, or more generally gravitational force, is the force between two
objects A and B. It is studied a long time ago by many famous scientists.
The most eminent person was Issac Newton. It was said that he had elaborated
his theory from the falling apple at the backyard. Of course that might just be
a fiction. But he had inspired the world towards the study of forces.
Among his three laws of motions, the second law is most related to the captioned
topic. F=ma, states that force is proportional to the mass of the object.
On a wider scale, there are forces acting on each others between any two objects.
So a more general form of the equation of force, F = GMn/r^2, was being
introduced to study the case.
It states that force F is proportional to the masses, M and m, of the two objects and
is inversely proportional to the square of the distance,r , between them.
What is the meaning of the statement in the above? A simpler way to understand
that may be as follows.
Suppose there are objects A and B, of mass M and m respectively. When M or
m gets bigger, keeping r unchanged, the force F will also be increased in a
uniform way. On the other hand, as the objects A and B are being moved apart
from each other, the forces exerted between them will be decreased.
After Newton, of course, people may be thinking of Einstein's theories. It's right
that Einstein's theory has really rectified many paradox that Newton had
left over. Also he has triggered another revolution of technology. But that shall be
another issue to be looking into.

In scientific terminology gravitation and gravity are distinct. "Gravitation" is the attractive influence that all objects exert on each other, while "gravity" specifically refers to a force which all massive objects (objects with mass) are theorized to exert on each other to cause gravitation. Although these terms are interchangeable in everyday use, in theories other than Newton's, gravitation is caused by factors other than gravity. For example in general relativity, gravitation is due to spacetime curvatures which causes inertially moving objects to tend to accelerate towards each other. Another (discredited) example is Le Sage's theory of gravitation, in which massive objects are effectively pushed towards each other.

我上面個答案都答唔倒咩叫地心吸力
Gravity is a force pull to the central of the Earth.