Astronomy – Book Notes
4.4 The Universal Law of Gravitation

Newton’s Three Laws of Motion – describe how objects in the universe move in response to forces.
What Determines the Strength of Gravity

Newton – Universal Law of Gravitation:
o
Every mass attracts every other mass through the force called
gravity
o
Strength of gravitational force attracting two objects is
directly proportional
to the product of
their masses.
Doubling mass of one object
doubles force of gravity between the two objects
o
Strength of gravity between two objects decreases with the
square
of the distance between
their centers.
Gravitational force follows an
inverse square law
with distance.
•
Doubling distance
weakens force of gravity by a factor of 4.
o
Mathematically: (force of gravitational attraction) = (Gravitational Constant)[[(mass of object
1)x(Mass of object 2)]/(distance between their centers – squared)]
How does Newton’s Law of Gravity extend Kepler’s Laws?
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 Spring '08
 HORNSTEIN
 Astronomy, Mass, General Relativity, Kepler's laws of planetary motion

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