1. Gravity and Orbits – Not Free Fall
What happens when something holds you up.
Gravity pulls you down
Floor pushes you up
Forces balance - you don’t go anywhere.
You know which way is up because that is the way the floor is pushing you

2. Gravity and Orbits – Free Fall
Cut the cable
The elevator starts falling
The floor no longer holds you up
You fall too
You float around – no gravity
Because you and elevator are falling at the same rate, feels like weightlessness
Free fall can be up, down, or sideways
Help!

3. 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

4. Q. 10: Why Is There “No Gravity” in Space?
Gravity and Orbits
Q. 10: Why Is There “No Gravity” in Space?

5. 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
If velocity is bigger, it leaves on a hyperbola

6. Two Body Orbits (M + m)P2 = a3 It does! Center of Mass
How come the Sun doesn’t go around the Earth?
It does!
Center of Mass
The Center of Mass is where the two masses would balance
The objects go around each other
(M + m)P2 = a3

7. Applies to planets around the Sun
Kepler Vs. Newton
Applies to planets around the Sun
P2 = a3
(M + m)P2 = a3
Always applies
Masses in Solar Masses
Sun + Earth
M + m =
 1

8. 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

9. Precession
Gravity pulls unevenly on the flattened Earth
Earth is like a top – its tilt changes over time
26,000 year cycle

10. 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

11. Tidal Forces

12. Q. 11: How Often Do Tides Occur?
Tidal Distortion
Greatly Exaggerated
Q. 11: How Often Do Tides Occur?

13. Spring (Big) and Neap (Small) Tides
Big Tides
Small Tides

14. Bulges and the Earth/Moon System
with rotation
without rotation
Earth
Earth
Moon
Q. 12: Tidal Locking
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

15. Tidal Locking
Moon
Earth
Moon
Moon
Moon

16. Tidal Heating planet moon Changing shape as distance changes
Flexing generates heat
planet
moon

17. Roche Limit
Planet
Moon

18. Q. 13: Titan and Tidal Forces
Roche Limit - Rings
Q. 13: Titan and Tidal Forces