Transcript of PHY134 Introductory Astronomy Tides and Matter 1.
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- PHY134 Introductory Astronomy Tides and Matter 1
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- Leftovers Earth is in free fall under gravity of Sun, so Suns
gravity has no effect on Earth! Almost none. There are remnants of
gravity even in freefall: tidal forces These are due to the fact
that gravitational acceleration is different at different points.
So not all points of an extended object can possibly be
simultaneously in free-fall Difference in free-fall acceleration
(from center of Earth) acts as a tidal force 2
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- How Strong is this Force? 4
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- What about the Moon? 5
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- The Tides Moon deforms water so bulge faces Moon. As Earth
rotates, bulge moves around Earth so tides repeat every 24h 48m
Earths rotation drags bulge East so lags Moon by about 12m Sun
exerts tidal force towards Sun about as strong. At full/new Moon
act together creating intense spring tides. At quarter Moon
counteract to create weak neap tides 6
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- Even More Tides When Moon formed molten and closer - Earths
tidal forces deformed it so it froze with permanent bulge. Tidal
forces keep this bulge aligned with direction to Earth: tidal
locking is why we always see same side of the Moon Since tidal
bulge on Earth is dragged East of Moon, tidal force of Moon tries
to align it. This in fact slows Earths rotation, transferring
angular momentum to the Moon which thus recedes into higher orbit
(G. Darwin, 1898) 7
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- What Now, Aristotle? Applying universal laws leads to unified
understanding of many phenomena! In space, everything is in
free-fall. Trajectories are Keplerian orbits. Internal structure
controlled by tidal forces is powerful. Learn more about matter and
forces 8
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- Matter By early 1900s: a unified understanding through atomic
theory All matter made of a hundred or so elements types of atoms
labeled by These bind to form molecular compounds Three states:
solid, liquid, gas Bulk properties determined by microscopic
dynamics Temperature is a measure of average random motion of atoms
and molecules In ideal gas measured in pressure in 9
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- States At low temperature and sufficient pressure form (almost)
incompressible liquid. Density decreases with temperature: hot
fluid rises In equilibrium with gravity pressure increase with
depth proportional to density In solid state positions of atoms
fixed maintain shape under external force Perturbations travel
through matter as sound waves with a speed characteristic of the
material We hear sound in air. 10
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- Waves Periodic disturbances characterized by frequency in
Traveling at speed produce periodic wave with wavelength Amplitude
is value of the perturbation at maximum Energy flux in carried by
wave is When two waves meet disturbances add If opposite sign -
subtract! 11
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- Doppler Effect Sound from a moving source heard at higher/lower
when source approaching/receding Doppler (1842) source receding
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- Heat Transfer An object hotter than environment will lose
energy until temperatures equilibrate Conduction: Heat can be
transferred through continuous contact. Rare in astronomy
Convection: Physical motion of fluid carries energy. Works well
when heating from below - Heated fluid less dense so rises
Radiation: Hot objects glow losing energy to light 13
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- Luminosity and Brightness Sun is hot so radiates energy at a
rate Luminosity in Brightness is flux in At a distance radiation
distributed uniformly on surface of sphere 14