Gravity is the general force of attraction between two objects with mass, independently of other forces. Earth not only has gravity, but the Moon, planets, stars and all other objects with mass in the universe have it as well.
In astronomy and cosmology gravitational forces explain many physical phenomena. It is this force that holds the moon and the planets in their orbits, it initiates the birth of stars and keeps them alive. It also is the responsible force of curious objects like neutron stars and blackholes and it even curves space and dilates time.
Fundamental
force.
Gravity is the weakest of the four
fundamental forces. However, it is a long-range attractive
force affecting all matter, that is why it is the most dominant
force in the universe. The strength of the gravitational force
between two bodies depends on their mutual distances; the larger the distance,
the smaller the force. Furthermore, it is dependent of the mass of the two
objects; the greater the mass the larger the gravitational force. Gravity
is barely noticeable in objects we see every day. However, when an object
has an enormous mass, as is the case of the Earth, the effect of this force is more noticeable. People are kept 'on the ground' and objects fall 'down',
because they are attracted toward the Earth by gravity.
Mass and
weight.
The concepts of mass and weight are not the same.
Every physical body has an inherent property called its mass. Mass
may most simply be considered as the amount of matter in any object.
Gravity gives us our weight, the larger the gravitational
force the 'heavier' we are. Weight is simply a measure
of the force of gravity acting on a body, and this varies depending on
the strength of that force. A flea weighs more on Earth than on our much less massive Moon.
That is why it could jump much higher on the Moon than on the Earth. Its mass,
however, will remain the same anywhere in the universe.
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In the universe there are many objects with a much larger mass than Earth. In
neutron stars, for instance,
all matter is so densely packed that their mass is billions of times
larger than the Earth's, and so is their gravity. On the surface of this type of
star the flea would weigh thousands of tons, and would be crushed
by its own weight and could not even attempt to jump.
Black holes are
even more dense, their gravity even pulls light back towards its surface.
 Escape
velocity.
The driving force of a launching rocket must be larger than the gravitational force that pulls it back toward Earth. This is because its rising speed is decelerated by the gravity on Earth. To escape from Earth, the escape velocity must be at least 40,000 km/h. |
Gravity
accelerates too.
When a force is acting on an object, the object will change its speed, in other words it is accelerated (or decelerated). Galileo Galilei deduced that gravity causes a definite continuous acceleration, rather than a velocity. He also stated that this acceleration is the same for all objects traveling in a vacuum. This means that the acceleration caused by gravity on an object is independent of the mass of that falling object. The gravitational attraction is the only significant force that is acting on the object. The acceleration of gravitational forces is often observed in the universe. For example, neutron stars emit röntgen radiation because the falling velocity of matter that is attracted by these stars is accelerated to one third of the speed of light. Thanks to the same phenomena we may observe black holes. |
The experiments of Galileo
Galilei. 
Galilei had balls of different weight rolling down from a hill
with only a marginal slope. He observed that all of the balls accelerated equally,
completely independenlty of their weight.
If a ball is rolled from a slope of one to ten meters, than the ball
will have a velocity of one meter per second after one second. It will continue to accelerate to two meters
per second after two seconds, and so on, no matter what the weight of the
ball may be. From this he concluded that the falling velocity of
all the objects are equal.
Determination of the planet orbits.
It was Isaac Newton,
in 1867 who published a universal theory to calculate the planet orbits.
His own (mentionedbelow) laws
gave Newton the idea that there are continuous forces acting on
the Moon and other planets, that describe circular orbits. Pondering the central force that is responsible for the motion
of the planets in their orbits, Newton came to the conclusion that an equal force causes objects to drop to Earth: gravity.
He perceived that it was this force, that also forces the Moon
to rotate around the Earth.
Suppose
a tank fires its gun. The ball will then be pulled by gravity and
'fall' towards the center of the Earth. However, the ball is also subject
to the forward forces of the gunshot, so it moves in a vertical
direction as well as a horizontal direction. This forces the canon ball to fly in a curved
line. When the tank uses more gunpowder, than the ball will get a higher
horizontal velocity and so it will fall farther away.
Now, Newton contrived that when the starting velocity of the
ball is high enough, than the ball will never fall back to
Earth, however it willgo into orbit around the Earth.
(hold your mouse over the image). His conclusion was that the Moon is constantly falling to Earth,
which results in its elliptical orbit.
On basis of the First law of Kepler, that planets follow elliptical orbits, as well calculations of acceleration on Earth caused by gravity developed by himself, Sir Isaac Newton calculated the motion of the Moon.
Consider the blue object to have a constant straight forward velocity,
moving from left to right. Now, due to gravity, the blue object
is 'falling' towards the center of red object, but it also continues
to moving straight forward. The result of this is that the
blue object is forced into a circular movement.(hold your mouse over the image to see the circular movement)
Universal
law of gravity.
On the above conclusion, Newton made his Universal law of gravity,
which stated that there is a gravitational attraction between every pair
of objects, directly proportional with the mass of each object and inversely
proportional to the square of the distance between them. From then on it was possible to predict the orbits of all planets
exactly.
The
force between two bodies is twice as strong when the mass of one of the
two bodies is doubled.
This is
logical, because you may see the doubling in weight of the body, as something
that is built out of two bodies of the original size. Each of these two
bodies would attract the other body with the original force. Therefore the
total force between the two bodies will be twice the original force.
If one of the objects would poss twice the mass, and the other one three
times, then the force will become six times larger.
Now we can understand why all bodies fall with the exact same velocity:
a body of twice the weight will be attracted with twice the force, but
it will also contain twice the mass. According to the second law of Newton,
these two influences will neutralize each other, therefore the acceleration
is the same.
The limitations
of Newton's Universal law of Gravity.
Although the law of Gravity is universal, still it appeares
to have several limitations. An example of this is the orbit of the planet Mercury; this planet
follows an elliptical orbit, where every complete rotation around the sun deviates with an constant
factor from the preceding orbit.
Because of the short distance of Mercury to the Sun, the mass forces
of the Sun are acting on it more than on the other planets, in such case
other laws are applicable.
The laws of Newton give no solution for this, the General
Theory of Relativity by Einstein however
predict this phenomena precisely.
The
three laws of motion.
In 1687 the book Philosophie Naturalis Principia Mathematica (De
Principia) was published by Isaac
Newton, in which he explained his three laws of motions.
They are a description of the mechanical influences on the motion of
objects on Earth, and also in the Universe. They read as follows:
1. Inertia (inertia is a body's resistance to acceleration):
An object remains in rest, or remains at a constant speed,
unless forces from outside are acting on it.
The ball in the experiments of Galilei was continuously influenced by the same force (its weight), resulting in the acceleration. Newton's conclusion from this was that the actual influence of the force was a continuous changing in speed of the ball, and not that the force was only put the ball in motion. Acceleration (and deceleration) seemed to be caused only by a force. This also meant that a body without influence of a certain force would stay in a constant linear motion.
2. Motion:
The change in speed of an object shall be equal in size
and direction to the force that act on it.
The greater the mass of a body is, the greater is its resistance to
acceleration. To state it another way, when the same amount of force is
applied to two different bodies, the force induces a smaller acceleration
in the body with the larger mass. The stronger the engine of a car, the faster it accelerates, but the heavier the car, the slower it accelerates (with the same engine).
3 Reaction:
Every action creates an equal opposite reaction.
This law is important for general physics, however is so relevant for
the determination of planet orbits.