Galileo Galilei and Isaac Newton both contributed to the discovery of gravity through experiments. Galileo conducted free fall experiments from the Leaning Tower of Pisa in which he showed that all objects fall at the same rate regardless of their mass. This disproved the then-prevailing theory. Newton later formalized the law of universal gravitation by demonstrating that gravity is what keeps the planets in orbit around the sun and causes objects to fall on Earth. He described gravity as a force that acts between any two masses.

1. The Source to our Existence

2.  Gravity is a force pulling together all matter (which is
anything you can physically touch). The more matter, the
more gravity, so things that have a lot of matter such as
planets and moons and stars pull more strongly.
 Mass is used to measure the amount of matter in something.
The bigger something is, the stronger its gravitation pull
becomes. As we walk on the surface of the Earth, it pulls on
us, and we pull back. But, the Earth has much more mass
than us, the pull from us is not strong enough to move the
Earth, where as the pull from the Earth can make us fall flat
on our faces.
 In addition to differences in the amount of mass an object
contains, gravity also depends on the distance of objects.
This is why we are held down to the surface of the Earth,
instead of being pulled off into the gravitation of Sun, which
is has hundreds of times the gravity of the Earth.

3.  Mass is used to talk about how much matter there is in something.
(Matter is anything you can touch physically.) On Earth, we weigh
things to figure out how much mass there is. The more matter
there is, the more something will weigh. Usually, the amount of
mass something has is related to its size, but that’s not always the
case. A balloon blown up bigger than your head will still have less
matter inside it than your head (for most people, anyhow) and
therefore the balloon has less mass.
 The difference between mass and weight is that weight is
determined by how much something is pulled by gravity. If we
are comparing two different things to each other on Earth, they
are pulled the same by gravity and so the one with more mass
weighs more. But in space, where the pull of gravity is very small,
something can have almost no weight. It still has matter in it,
though, so it still has the same mass.

4.  Gravity brings us back down to the ground when we
jump up. It stops us from floating into space.
Gravity keeps our food on our plates and our drinks
in our glasses.
Gravity keeps the Earth in orbit around the sun and
keeps us warm.
Gravity also keeps the water on the Earth, the
inhabitants on Earth, and the Earth's atmosphere in
its place.
Gravity pulls rain and snow down to our rivers.

6.  I begins with the famous story of Sir. Isaac
Newton. He was hit in the head with an apple
which fell from an apple tree. This inspired
him to question why the apple fell. On 1687
Newton states his Law of Universal Gravity,
which states that there is a gravitational force
acting between any two objects in the universe
at any given time.

7.  Galileo Galilee also contributed to the
discovery. Before Newton's discovery of
gravity, Galileo conducted experiments which
showed that all objects fall at the same rate,
regardless of their mass. The name of the
experiment was the free fall experiment. In this
experiment Galileo dropped two objects of
different masses off of the Leaning tower of
Pisa. This illustrated the fact that objects of
different masses fell at the same rate of speed.

8. WHO DISCOVERED THE
UNIVERSAL LAW OF GRAVITY
WHO DISCOVERED THE
CONCEPT OF FREE FALL

9.  http://www.google.com/imgres?imgurl=http://static.duartes.org/img/blogPosts/galileo-
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 http://scienceprojectideasforkids.com/2009/sir-isaac-newton/
 http://ellerbruch.nmu.edu/classes/cs255f03/cs255students/jedens/p4/factpg4.html
 http://www.juliantrubin.com/bigten/galileofallingbodies.html
 http://www.qrg.northwestern.edu/projects/vss/docs/space-environment/2-what-is-mass.html
 http://www.qrg.northwestern.edu/projects/vss/docs/space-environment/1-what-is-gravity.html
 http://www.youtube.com/watch?v=H0pGEq7bhLM&feature=fvw