Gravity is really an unknown force. We
can define it as a field of influence, and
that it effects the entire existence of the
universe. Some people think that gravity
consists of particles called gravitons,
which travel at the speed of light. The
only thing we do know is how gravity
operates in different parts of our universe.

Without gravity, there would be no space
and time.

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There is a legend that says that Galileo
once dropped two objects off the Leaning
Tower of Pisa to show that the heavier of
the two objects
dropped faster. If a feather and hammer
were the two objects he used then
obviously the hammer would hit the
ground first. This is due to air resistance,
which is the force air exerts on a moving
object. This force acts in the opposite
direction to that of the object’s motion. In
the case of a falling object, air resistance
pushes up as gravity pulls down, which
causes the object to slow down. When
Galileo’s experiment was repeated on the
moon, the hammer and the feather hit the
ground at the exact same time. This is due
to
the fact that the moon has no atmosphere.

Therefore, air resistance doesn’t exist on
the moon. Also, the amount of air
resistance on an object depends on the
speed, size, shape, and density of the
object. The larger the surface area of the
object, the greater the amount of air
resistance on it. This is why feathers,
leaves, and sheets of paper fall more
slowly than pennies, acorns, and
crumpled balls of paper.
There is another legend that states that
when Newton was lying against a tree in
an orchard, he was struck on the head by
an apple. He wondered what provided the
acceleration for the apple to fall to the
ground. Was this a force of the earth on
the apple? If so, then the apple must exert
a force on the earth according to
Newton’s law of action/reaction forces.


Newton applied this theory unto the
planets, which orbit the sun. He found by
studying astronomical data, that the force
that held the earth in orbit around the sun
was the same force that drew the apple
toward the earth. This was the
force of gravity that is given by this scary
formula:
F = Gm1*Gm2
gravity _______
r^2
F equals the force in Newtons, G equals
the gravitational constant which is 6.67 *
10^-11 Nm^2 | kg^2, m1 and m2 equal
the mass of each body in kilograms, and
finally, r equals the distance between the
2 bodies in meters.


If all of this is confusing, I feel your pain,
because it took me a long time to get this
all down!
Another concept that is important to
understand is terminal velocity. Terminal
velocity is the highest velocity that will be
reached by a falling object. As an object
falls through air, air resistance gradually
increases until it balances the pull of
gravity. According to the law of inertia,
when the forces acting on an object are
balanced, the motion of the object will not
change. When this happens, the falling
object will stop accelerating. It will
continue to fall, but at a constant, final
velocity.


Newton’s laws of motion and law of
gravitation can be used to explain the
forces, position and motion of all objects
in the universe. A simple
analogy of how gravity controls the
motion of a planet around the Sun can be
shown by a mass on the end of a string
being spun around in a horizontal
plane at constant speed. The ball has
constant speed but the direction is always
changing so according to the definition of
velocity the object must be undergoing a
constant acceleration. According to
Newton’s second law, for a mass to be
accelerating, it must have a resulting force
acting upon it. The question is: Where
does this force come?
The forces involved can be examined by
considering what happens when the string
breaks.


When the string breaks the mass is no
longer constrained to travel in its circular
orbit and moved off in the direction as
shown. This indicates that there must be a
force holding the mass in its circular orbit.

It is directed towards the center of the
circle and is called the centripetal force.

The centripetal force has a resulting
centripetal acceleration. The thing is, you
can extend all of these concepts, and
apply them onto the objects like the sun,
moon, planets, and even entire galaxies.

The gravitational force of the sun, acting
on the earth, keeps the earth in its orbit,
preventing it from traveling away into
interstellar space. The gravitational force
of the earth, acting on us, holds us to the
earth’s surface. The gravitational
attraction between a person
and the earth is proportional the person’s
mass and inversely proportional to the
square of the planet’s radius (distance
from the person to the center). This
number for gravitational attraction is
called your weight.
Every planet has mass and so every
planet exerts a gravitational force on
nearby objects. We say that planets have
gravity. However, what we really mean is
that there is a gravitational force of
attraction between the planet and a person
standing on the planet’s surface. This
force depends on the visitor’s mass, the
planet’s mass, and the planet’s radius.

Accordingly, people have different
weights on different planets.
For example, a person on the moon
weighs only about 1/6 as much as on
earth. The moon’s radius is 25% earth’s
radius and the moon’s mass is 8% of
earth’s mass. So, if a student weighs 150
pounds on earth, they would weigh only
(1/6) * 150 pounds, which equals 25
pounds, on the moon.


Gravity does more than just keeping
planets orbiting the sun and causing
people to have weight, gravity also causes
tides. In simple terms, the tides
are caused by the gravitational attraction
between the moon and earth’s oceans
AND by the motion of earth through
outer space.


Einstein predicted gravitational waves.

They are best understood in comparison
with electromagnetic waves, which were
predicted by Maxwell in 1864 and
discovered by Hertz 22 years later.


Hertz discovers electromagnetic waves in
1886. Electromagnetic waves are waves
of electricity. They give us our sense of
vision with which to see the universe.

Gravitational waves are waves of gravity.

They are vibrations of space itself. They
travel through space at the speed of light,
but are more like sound than light.


Hertz’ discovery set the foundation for
the electronic revolution of the twentieth
century. Electromagnetic waves not only
revolutionized our lives, but also our
knowledge of the universe. Astronomers
gradually opened the electromagnetic
spectrum, first using visible light and then
radio, x-rays and gamma rays. Each new
part of the spectrum provided us with
dramatic new insights into the universe.


Einstein predicted gravitational waves in
1916. They have not yet been directly
detected on earth, although astronomers
Joe Taylor and Russell Hulse received the
1993 Nobel Prize for proof of their
existence, by showing that a
star system is losing energy by producing
gravitational waves.


Gravitational waves are a completely new
spectrum. If electromagnetic waves let us
see the universe, gravitational waves will
let us hear the universe. They will provide
us with a new sense, the sense of hearing,
with which to explore the universe.
Gravity is a very complicated subject, but
scientists are learning more and more
about it as time goes on. Contributions
from people such as Newton and Einstein
helped shape the way we see things
today. Without them, no telling what kind
of misconceptions we all might believe in
today.