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 Force of gravity = a constant x (distance between objects) 2 (mass of object 1) x (mass of object 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

7 (see ch 5 cannon ball applet)

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

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

10 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)

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

12 energy Kinetic energy energy of motion 1/2 x mass x (speed) 2 1/2 m v 2 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: (sigh) (mass of object 1) x (mass of object 2) distance between objects PE = a constant x

13 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?

14 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?

15 energy conserved (KE + PE = constant) Example: sun and planet 1/2 m planet v 2 + G (m sun m planet )/d = constant

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

17 energy + atoms + gravity + angular momentum = Solar System formation 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 Yay! what happens to the speed of its atoms? the atoms move faster fasterincreases

18 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

19 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

20 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