Newton's Law of Universal Gravitation
Apples had a significant contribution
to the discovery of gravitation. The
English physicist Isaac Newton
(1642-1727) introduced the term
"gravity" after he saw an apple
falling onto the ground in his garden.
"Gravity" is the force of attraction
exerted by the earth on an object.
The moon orbits around the earth
because of gravity too. Newton later
proposed that gravity was just a
particular case of gravitation. Every
mass in the universe attracts every
other mass. This is the main idea of
Newton's Law of Universal
A portrait of Issac Newton.
Courtesy of AIP Emilio Segre Visual
Archives, W.F. Meggers Collection.
The law was published in Newton's
famous work, the Principia
("Mathematical Principles of Natural
Knowledge") in 1687. It states that every
particle in the universe exerts a force
on every other particle along the line
joining their centers. The magnitude
of the force is directly proportional
to the product of the masses of the
two particles, and inversely
proportional to the square of the
distances between them.
In mathematical terms:
By team C007571, ThinkQuest2000.
where and are the masses of the two
r is the distance between the two
F is the gravitational force between
G is the universal gravitational
The above equation only calculates the
gravitational force of the simplest case
between two particles. What if there are
more than two? In that case, we
calculate the resultant gravitational force
on a particle by finding the vector sum of
all the gravitational forces acting on it:
By adding the unit vector to the
equation, F now processes a direction!
Interactively test the effects of
gravitation on planets!
Newton derived the relation in such a
way that F is proportional to m
because the force on a falling body
(remember the apple?) is directly
proportional to its mass by Newton's
2nd law of motion: F...
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