Download Document

Lecture series from Conceptual Physics, 8th Ed.
Copernicus, Brahe, and Kepler
p142
Used Brahe’s data to
develop mathematical
models of the motions
of the planets.
Speculated that the sun
was the center of the
solar system.
Thought he would gather some
data before shooting off his
mouth about his heliocentric
ideas. Which was probably a
good idea seeing as how his
nose had already been cut off
in a duel. (He was rich and threw
really gnarly parties so he got away with
a lot.)
Kepler’s Laws p143
First he translated Brahe’s data from Earth based reference to
what would be seen by a stationary observer outside the solar
system.
His laws of planetary motion are:
1. Planets move in ellipses.
2. The “equal areas” thing.
3. T2 proportional to R3
Fantastic stuff, but he didn’t know about gravity, didn’t
apply “inertia”, didn’t know why ellipses…
BUT, he was right because he had good data.
Newton’s Law of Universal Gravitation
p144
-knew that inertia kept the planets moving.
-the force acting on the planets caused them to curve.
-from looking at Kepler’s stuff, he knew the force
emanated from the sun. (He deduced gravity.)
-he also deduced that the forced decreased with
the square of the distance.
-the apple thing reminded him that gravity acts
between masses and therefore objects in orbit are
falling around their primaries.
Here it is gang!!
Gm
m
1
2
Fg =
d2
The Universal Gravitational Constant, G
G=
F d2
m1m2
p145
= 6.67x10-11 Nm2/kg2
An interesting example:
How much does the earth mass?
Imagine an object on the surface of the earth.
From F=Gmemo/d2
So, me =
mog = Gmemo/d2
g d2
(9.8) (6.4x106)2
=
G
(6.67x10-11)
= 6.0x1024kg
Gravity and Distance: The Inverse-Square Law p147
Weight
decreases
as person
climbs.
Weight and Weightlessness p149
Fig. 8.7 Your weight
equals the force with
which you press against
the floor.
More Weightlessness
Both people are in
a gravitational
field.
In fact. Gravity does
not produce weight!
The floor produces
weight.
Ocean Tides
p150
Equal forces to the right
means Jell-O ball stays
spherical.
Forces shown here cause left
side to lag behind.
The left side is further from
moon and is not pulled as
strongly toward moon.
More on Tides
F = G m m / d2
This planet has bigger tides because “delta” F
is greater.
Smaller “delta” F means
smaller tides.
“Delta” F refers to the difference in
forces at the front and back of the
planet.
Spring tides:
Neap tides:
The earth’s tilt means that this person has a
lower high tide than this person.
Tides in the Earth and the atmosphere
work the same way.
Tides on the Moon
p154
Why does the moon always
face us?
The key thing is that the moon acts
like a pendulum.
This part of the moon
swings back and forth
trying to line up with
the Earth.
The Gravitational Field Inside a Planet
First, the outside
field lines:
Now, the inside:
Last, the in and the out -side
p155
Einstein’s Theory of Gravitation
p157
Albert says that the moon is
NOT pulled to the Earth.
The moon curves because the
surface it travels on is tilted
toward the Earth.
Black Holes
p157
Their gravitational field is so strong that even light can not escape!
Because of F=Gmm/d2
When the star collapses to ½ its
radius, Supy’s weight quadruples.
A collapsing star with
Superman standing on its
surface.
Only gravity, charge and
angular momentum
remain.
When the star crushes its matter to
a few hundred kilometers, that may
be the last we see of it.
Things orbit it
and spiral
inward.
Universal Gravitation
p 159
As far out as we can see, matter
seems to obey Newton’s law of
gravitation: F=Gmm/d2.
Neptune and Pluto
were discovered by
using the law.
The activities of distant galaxies
seem to obey the law.
BUT, the universe is expanding too fast!!
To be continued.