a little physics • SESAME Astronomy Winter 2011 • week 2
helpful concepts • mass • gravity • angular momentum • atoms • heat/energy/temperature
mass • “stuff” • matter • inertia
Gravity • force between 2 objects with mass • always attractive • increases with increasing mass • decreases with increasing distance between objects (mass of object 1) x (mass of object2) Force of gravity = a constant x (distance between objects)2
Gravity is responsible for... • Keeping us grounded • Keeping the moon in its orbit around Earth • Keeping the planets (and asteroids) in their orbit around the sun • Keeping the sun in its orbit around the Milky Way • making comets fall in toward inner solar system • making gas clouds collapse into stars and planets • Putting Andromeda and the Milky Way on a collision course • Making us fall into the local Supercluster • creating the anisotropies in the cosmic microwave background • keeping stars together so they can burn (instead of blowing apart from their heat
(see ch 5 cannon ball applet)
angular momentum • moment of inertia x angular speed (how fast it’s turning) • it’s conserved • big and spinning slowly = small and spinning quickly
atoms • building blocks of matter • smallest unit of an element that still has properties of that elements • made of protons, neutrons, and electrons
atoms • different elements have different numbers of protons (and neutrons, and electrons) • 1 atomic mass unit (amu) = mass of 1 proton (or neutron) • ignore electron’s mass (1/1700 amu)
atoms • protons -> positive charge • neutrons -> no charge • electrons -> negative charge • nucleus (center) • protons and neutrons • where most of mass is
energy (sigh) • Kinetic energy • energy of motion • 1/2 x mass x (speed)2 • 1/2 m v2 • Potential energy • has the potential to have kinetic energy • equation depends on what’s generating PE • gravitational PE on surface of Earth: mass x constant x height • general gravitiational PE: (mass of object 1) x (mass of object 2) PE = a constant x distance between objects
energy • imagine a bowling ball and a golf ball moving at the same speed. which one has higher kinetic energy? • imagine a bowling ball and a golf ball with the same kinetic energy. which one is moving faster?
energy • imagine a Hydrogen atom (1 proton, 0 neutrons) and a Carbon atom (6 protons, 6 neutrons) at the same temperature. which one is moving faster?
energy • conserved (KE + PE = constant) • Example: sun and planet • 1/2 mplanet v2 + G (msun mplanet)/d = constant
energy + atoms = temperature • Temperature = Kinetic Energy of atoms • comparisons: • at a given T, are more massive atoms are moving faster or slower?
energy + atoms + gravity + angular momentum = Solar System formation Yay! • imagine a gas cloud (mostly Hydrogen, some Helium, a little Carbon, Oxygen, Nitrogen, Iron...) • it’s rotating slightly • gravity pulls the atoms in - makes it collapse • as the cloud contracts, • as atoms move , temperature what happens to the speed of its atoms? the atoms move faster faster increases
energy + atoms + gravity + angular momentum = Solar System formation • some of the atoms stick together - now they have more mass, so they attract more atoms, which gives them more mass, and planets form • the center of the cloud is the densest hottest part, and it gets so hot that fusion starts in the center - a star is born! we’ll get to fusion when we cover stars, or at least, the sun
energy + atoms + gravity + angular momentum = Solar System formation • closer than a certain distance, just inside the orbit of Jupiter, the temperature is relatively high, so light elements (Hydrogen and Helium) can escape • same T, lower, mass -> higher speed (in fact, escape speed) -> rocky (terrestrial) planets • Farther out, the light elements can’t escape/evaporate, so they get accreted onto the planets -> gas (jovian) planets • smaller rocky planets in inner Solar System, larger gas planets in outer Solar System
ch 9 • collapse of solar nebula (with and without skater), • formation of protoplanetary disk • why does the disk flatten • accretion and formation of planets, • condensate regions • comparative planetology • ch 8: • kepler_3_orbit...htm
