Physics Chapter 9 - Gravity
Newton and Gravity
So this guy Newton…

Devised three laws of
motion.
 Realized that all
objects accelerate
towards the ground at
the same rate, g.
 Recognized the
presence of some
force that kept the
“heavens” moving in
their elliptical paths.
Quick Review of Newton’s
discoveries…

Laws




1st Law – The Law of Inertia
2nd Law – acceleration = Force  mass
3rd Law – Forces occur in action-reaction pairs
Acceleration due to gravity, g


All objects regardless of their mass are accelerated
equally by gravity
Since this acceleration is so important to the motion
of all objects on the surface of the earth it is
assigned g = 9.8 m/s2 (or 10 m/s2)
Newton & Gravity

Isaac Newton was the first to
deduce that the force that makes
an apple fall is the same force
that keeps the moon in its orbit.
 His law of gravitation states that
every object pulls on every other
object based on their mass and
distance between them
 But then he asked himself…


Does the falling apple exert a force
on the earth?
Why doesn’t the moon fall into the
earth?
Answers…

Apple DOES exert a force on the Earth!


The moon IS falling!


Does the Earth notice? Why?
HUH?!?
Newtonian Mountain
Newton’s Universal Law of
Gravitation
What do you think it depends on?
Newton’s Universal Law of
Gravitation

Depends on…

Mass of the two attracting objects (m1 & m2)


Distance between the two objects (d)


Directly proportional
Inversely proportional
Universal Gravitation Constant (G)

G = 6.67 x 10-11 Nm2/kg2
m1m2
F G 2
d
Example #1
A 10,000 kg asteroid is close to a 20,000
kg asteroid. If their centers are 100 m
apart, what is the force of gravity between
them?
The Story of 9.8…

From before we know that a 1 kg
mass exerts a force of 9.8 N on the
Earth because…
Fweight = mass x gravity = (1kg)(9.8 m/s2)

Where did this “9.8” come from in the
first place?
The Story of 9.8…
1 kg mass
distance = radius
Earth



mobject mEarth
m1m2
F  G 2 becomes... F  G
d
rEarth 2

F  mobject 6.67 1011
F = mobject x 9.8

5.97 1024
6.3810 
6 2
Acceleration due to Gravity at
Different Places on Earth…
g
varies slightly due to differences in
the distance from the center of the
earth (altitude or latitude)
North Pole
 Panama Canal
 New York City
 Denver

g = 9.832 m/s2
g = 9.782 m/s2
g = 9.803 m/s2
g = 9.796 m/s2
Acceleration due to gravity on
Different Planets…
These depend on
the planet’s mass
and radius.
Jupiter
Neptune
Saturn
Uranus
Earth
Venus
Mercury
Mars
Pluto
Moon
2.54 x g
1.20 x g
1.06 x g
0.92 x g
1.00
0.92 x g
0.38 x g
0.38 x g
uncertain
0.16 x g
Newton and Gravity Questions
1.
When do you weigh more; at the bottom of Death
Valley, or atop the high Sierras? Why?
2.
What is the magnitude of gravitational force between
two 1-kilograms objects that are 1 meter apart?
3.
What do we call the gravitational force between the
Earth and your body?