Gravity
Orbits
How come the astronauts float around when they are inside
the space shuttle?
A) The shuttle’s rockets compensate for gravity
B) The shuttle is so far from earth that gravity is very, very
weak.
C) Earth’s atmosphere shields the shuttle from gravity
D) The Moon’s gravity compensates for the Earth’s
E) NASA didn’t pay the gravity bill
F) There is nothing holding the shuttle up
Gravity and Orbits - Not Free Fall
•Cuthappens
the cablewhen something holds you
What
up.•The elevator starts falling
•The
floor
no
longer
holds
you
up
Gravity
pulls
you
down
Help!
•You
too you up
Floorfall
pushes
•You float
around
no gravity
Forces
balance
- you– don’t
go anywhere.
•Because
you and
elevator
are falling
You
know which
way
is up because
thatatis
same
rate, feels
like weightlessness
thethe
way
the floor
is pushing
you
•Free fall can be up, down, or sideways
Gravity and Orbits - Circular Orbits
“The knack [to flying] lies
in learning how to throw
yourself at the ground and
miss” - The Hitchhiker’s
Guide to the Galaxy
trilogy, in Life, the
Universe, and Everything
GM
v 
R
2
o
Gravity and Orbits
How come the astronauts float around when they are inside
the space shuttle?
A) The shuttle’s rockets compensate for gravity
B) The shuttle is so far from earth that gravity is very, very
weak.
C) The Earth’s atmosphere shields the shuttle from gravity
D) The Moon’s gravity compensates for the Earth’s
E) NASA didn’t pay the gravity bill
F) There is nothing holding the shuttle up
Non-circular orbits
•If velocity is more or less than this,
you get an ellipse
•If velocity is big enough, it
leaves on a parabola –
escape velocity
2GM
v 
R
2
e
•If velocity is bigger, it
leaves on a hyperbola
GM
v 
R
2
o
Two Body Orbits
•How come the Sun doesn’t go
around the Earth?
•The Center of Mass is where
the two masses would
balance
•The objects go around
each other
(M + m)P2 = a3
It does!
Center of Mass
Announcements
Date
Today
Monday
Tuesday
Read
Sec. 4.1 – 4.4,
Gravity Lecture
Sec. 5.1
Sec. 5.2 – 5.3
Test next Thursday!
Information sheet
online soon?
5/28
Quiz
yes
yes
yes
Kepler vs. Newton
P2
=
a3
(M + m)P2 = a3
Applies to planets
around the Sun
Always applies
•Masses in Solar Masses
Sun + Earth
M + m = 1 + .000003  1
Gravity vs. Pressure = sphere
•Gravity wants to squish everything to a point
•Pressure wants it to spread out
•They balance to make a sphere
•If an object rotates, it makes a flattened
(oblate) sphere
•Small objects can be any shape
Precession
•Gravity pulls unevenly on the flattened Earth
•Earth is like a top – its tilt changes over time
26,000 year cycle
Gravity vs. Rotation = Disk
Dust or gas or chunks - disk forms
Rotation keeps it from falling in
Gravity keeps it from escaping
•Rings around Planets
•Spiral Galaxies
•Accretion disks
•Protoplanetary disks
Tidal Forces
Tidal Distortion (greatly exaggerated)
Spring (big) and Neap (small) Tides
Sun
Big Tides
Earth
Moon
Earth
Sun
Small Tides
Moon
Tides
Generally, high tide occurs twice every day. Why?
A) There are two causes of tides, the Sun and the Moon
B) The Moon causes two places on the Earth’s oceans to
bulge and have high tide
C) The second high tide is sort of an echo of the first tide
D) The Moon goes around the Earth twice every day
Bulges and the Earth/Moon system
without
with rotation
Earth
Moon
Earth and Moon bulge want
to align
Strong effect on the Moon
Rotation causes bulge to be in wrong place
Bulge has gravitational pull on Moon
Earth rotation slows
Moon slung to higher, slower orbit
Earth
Moon
Moon
Why can we only see one side of
the Moon?
A) Because the Sun only shines
on one side of the Moon
B) Because the Moon doesn’t
rotate, so we never see the back
side of the Moon
C) The Moon rotates at the same
rate as it goes around the Earth
D) It is like a rainbow; it doesn’t
have another side
Moon
Tidal locking
Moon
Tidal Heating
Changing shape as distance changes
Flexing generates heat
planet
moon
Roche Limit
Planet
Moon
Roche Limit - Rings
Roche Limit - Rings
Where is Titan [a very large moon of
Saturn] compared to Saturn’s rings?
A) It is closer than the rings
B) It is farther away than the rings
C) It is within one of the rings
D) It is between rings - there are rings
both inside and outside it.