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a little physics

SESAME Astronomy Winter 2011week 2

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helpful concepts

mass

gravity

angular momentum

atoms

heat/energy/temperature

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mass

“stuff”

matter

inertia

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Gravityforce 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)

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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

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(see ch 5 cannon ball applet)

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angular momentum

moment of inertia x angular speed (how fast it’s turning)

it’s conserved

big and spinning slowly = small and spinning quickly

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atoms

building blocks of matter

smallest unit of an element that still has properties of that elements

made of protons, neutrons, and electrons

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atomsdifferent 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)

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atomsprotons -> positive charge

neutrons -> no charge

electrons -> negative charge

nucleus (center)

protons and neutrons

where most of mass is

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energyKinetic 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:

(sigh)

(mass of object 1) x (mass of object 2)distance between objects

PE = a constant x

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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?

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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?

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energy

conserved (KE + PE = constant)

Example: sun and planet

1/2 mplanet v2 + G (msun mplanet)/d = constant

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energy + atoms = temperature

Temperature = Kinetic Energy of atoms

comparisons:

at a given T, are more massive atoms are moving faster or slower?

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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

faster increases

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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

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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

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ch 9collapse of solar nebula (with and without skater),formation of protoplanetary diskwhy does the disk flattenaccretion and formation of planets,condensate regionscomparative planetology

ch 8: kepler_3_orbit...htm