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a little physics

This article explains fundamental concepts in physics such as gravity, angular momentum, atoms, and energy. It covers topics like mass, force of gravity, kinetic energy, potential energy, and the formation of the Solar System.

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a little physics

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  1. a little physics • SESAME Astronomy Winter 2011 • week 2

  2. helpful concepts • mass • gravity • angular momentum • atoms • heat/energy/temperature

  3. mass • “stuff” • matter • inertia

  4. 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

  5. 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

  6. (see ch 5 cannon ball applet)

  7. angular momentum • moment of inertia x angular speed (how fast it’s turning) • it’s conserved • big and spinning slowly = small and spinning quickly

  8. atoms • building blocks of matter • smallest unit of an element that still has properties of that elements • made of protons, neutrons, and electrons

  9. 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)

  10. atoms • protons -> positive charge • neutrons -> no charge • electrons -> negative charge • nucleus (center) • protons and neutrons • where most of mass is

  11. 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

  12. 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?

  13. 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?

  14. energy • conserved (KE + PE = constant) • Example: sun and planet • 1/2 mplanet v2 + G (msun mplanet)/d = constant

  15. energy + atoms = temperature • Temperature = Kinetic Energy of atoms • comparisons: • at a given T, are more massive atoms are moving faster or slower?

  16. 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

  17. 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

  18. 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

  19. 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

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