1. Gravity
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
(gravity, magnets,
electricity…)
2. Newton’s Math Proof
Force of gravity must Gravity must be
decrease by the proportional to both
inverse square of the masses because of
distance. Newton’s 3rd Law
F α 1/r2 (Equal and opposite
forces)
This is known as the
inverse square law! Fα Mm Combine
It is repeated in These!
electricity and
Mm GMm
magnetism! Fα 2 F= 2
r r
3. Law of Universal Gravitation
GMm G = gravitational constant (small or large?)
F= 2 G = 6.67 x 10-11 Nm2/kg2
r
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.
It took 100 years later for this law to be proven.
Henry Cavendish performed the experiment.
See next slide for a video
4. Video of Cavendish Experiment
Click on this link. You do not have to watch the
whole video. You can forward the tab to 8:10 and
watch up to 11:32 seconds. This will show you
where G from F = Gm1m2/r2 comes from.
•http://video.google.com/videoplay?
docid=7865632513511665648
•This second link will show you the meaning
of the Law of Gravitation.
•http://video.google.com/videoplay?
docid=3265790422523131277 view 13:00 to
5. 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 At Crown Point, the radius
object that affects of the Earth is 6.37101 x
other objects. 106 m. If the Earth has a
mass of 5.98 x 1024 kg.
Gravitational Field
Then calculate the Earth’s
Area around a gravitational field in Crown
mass that affects Point.
other masses
GM 6.67 x10−11 (5.98 x1024 )
What represents the g= 2 =
r (6.37101x106 ) 2
gravitational field around the
g = 9.83m / s 2
earth?
Editor's Notes
Show picture of Henry Cavendish’s experiment on the board. 1700’s
Draw picture of girl and boy on park bench (bubble “physics”) F = 1x 10 -6 N
