Newton rationalized that if the earth puts a force
on an apple, then who’s to say that it doesn’t
exert a force on the moon?
Contact vs. Non-contact Forces
↓ Newer concept: force
Must be touching acting at a distance
2. Gravity must be
proportional to both
masses because of
(Equal and opposite
F α M m
Newton’s Math Proof
Force of gravity must
decrease by the
inverse square of the
F α 1/r2
This is known as the
inverse square law!
It is repeated in
3. Keeping Relationships Simple
• Proportions: Compare two quantities while their
outcome remains constant. Symbol: α
• Inverse relationship means you are comparing
two quantities and as one quantity gets larger
the other gets smaller.
• F = ma
• When the F is kept constant the m is inversely
proportional to the a.
4. Still Keeping it Simple
Inverse Square relationship means that as
one quantity gets larger the other gets
smaller by the square of the smaller
F α 1/r2
When r goes up, F goes down.
5. Keep is Simple Direct
• Direct Relationship means that as one
quantity gets larger the other gets larger
• F α a
• As F increases so does the a.
• F = ma, mass will be kept constant.
6. Law of Universal Gravitation
Every particle in the universe attracts every other
particle with a force that is proportional to the
product of their masses and inversely
proportional to the square of the distances
between them. This force acts along a line
joining the 2 particles.
G = gravitational constant (small or large?)
G = 6.67 x 10-11
It took 100 years later for this law to be proven.
Henry Cavendish performed the experiment.
7. Sample Problem
A 50 kg girl & a 70 kg boy are 50 cm apart on a
park bench. Find the force of attraction…
Field Area around an
object that affects
Area around a
mass that affects
What represents the
gravitational field around the
At Crown Point, the radius
of the Earth is 6.37101 x
m. If the Earth has a
mass of 5.98 x 1024
Then calculate the Earth’s
gravitational field in Crown
2 6 2
6.67 10 (5.98 10 )
(6.37101 10 )
GM x x
g m s
Hinweis der Redaktion
Show picture of Henry Cavendish’s experiment on the board.
Draw picture of girl and boy on park bench (bubble “physics”)
F = 1x 10-6 N