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

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!

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

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

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

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

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 + .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

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

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

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

Tidal Locking Moon Earth Moon Moon Moon

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

Roche Limit Planet Moon

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