Dr. Henry Throop / University of Colorado ASTR 1110 Rocks: Geology and the Terrestrial Planets Announcements ------------- Observing night tomorrow HW#1: mostly good; 90% & 30% bimodal. Last problem: got right, but thought too hard! Ok to write times in useful units: 10 yrs is better than 3e8 s Esp. if problem suggests it! ---- Today: Also: Newton's & Kepler's Laws Tour of the planets Also: eclipses & things Also gravity Also phases of the planets & earth All these laws are 100% universal! Work for essentially anything orbiting anything else! They're not descriptions of our SS. Not measurements, not predictions of one particular case, or which depend on one size -- but rather, they're universal and work for every single system in the universe. Newton's Laws #1 At rest stays at rest, and in motion straight line until a force Demo: cabbages sitting on table. Does this sqaure with our knowledge? No! It's not obvious! There's always friction! #2 Force accelerates it in that direction Gravity of sun pulls planets into orbits. If it weren't, they'd be straight lines Demo: #3 Equal & opposite reaction Sun pulls jupiter around -- but Jupiter also pulls sun around Rocket doesn't `push' on anything. `Forcing' air out one side will force us the other way! Acceleration: Any change in speed or direction Kepler's Laws #1 -- Orbits are ellipses with Sun at one focus Not circles, but ellipses! #2 -- Equal areas in Equal times The closer you get to the sun in your orbit, the faster you go! #3 -- (orbital period in years)^2 = (average distance in AU)^3 or (orbital period in seconds)^2 = 4 pi^2 (average distance in cm)^3 / (G (m1+m2)) First form: works for our SS. But if our sun was heavier, it has a stronger force of gravity. Second form is totally universal: works for star orbiting a galaxy, or Moon orbiting Jupiter, or Charon orbiting Pluto, or distant world orbiting its own star, or spacecraft (NEAR) orbiting an asteroid. So, second is universal, and first is a handy form that works for our SS. Table: plot of p^2 v. a^3 Table: eccentricities: they're mostly low Gravity ------- F = G M1 M2 / R^2 Gravity pulls all things in SS together. Works stronger on heavier things, and weaker on things further apart. As far as we can see, gravity affects every single thing in the universe, and laws of physics are identical in other galaxies, and our SS. vg: other galaxy, with stars orbiting nucleus. Q: why does Earth go around Sun, not Jupiter? A: Earth _is_ affected by Jupiter, just not very much Major Q: why do planets go in orbits around sun? A: Gravity pulls them that way Demo: just like weight on a string. Let go, and goes in straight line. Q: Why does is so happen that force of gravity exactly cancels force of planet moving around? A: If it's not, planet would be at a different location! Gravity is a force that pulls the planets into their orbits! Without gravity, the planets would go in straight lines! Planets are falling in to the sun, just like an elevator is falling to the earth. Difference is that one starts from a `running start', and other is just dropped from a stop. Q: Might ask, why do these cancel each other exactly? A: if they didn't, planet would fall into sun! And a lot of them did! If I pull too weakly on string -- or too strongly -- then the planet gets lost. So, there's probably plenty of planets that have hit the sun since going too slowly, or have been lost out since going too fast. It's just these nine planets that remain. Go thru several applications of each law: Planetary orbits: free fall Comets Tides Tidal breakup Galaxies: missing mass Tidal Forces ------------ Tidal Breakup: Application of Newton's Law of Gravity _and_ Kepler's 3rd law: Force of gravity is stronger (Newton's law of gravity) Orbital period is faster -> If we have a large, weak body (e.g., giant snowball), inside part will orbit faster than outside part, and thus gravity will `tear apart' the body. vg: SL9; crater chain images Tides: Application of Newton's law of Gravity: Gravity is a bit stronger on the water than on the rock, since water is closer. Thus it gets pulled toward Earth. And every time Earth rotates underneath it, we get a high tide. If Sun is pulling on water _and_ the moon, tide is extra high! NB: slooshing of water is actually slowing down the Earth! Geology ------- Collab Q's ---------- If distance doubles, what happens to orbital period? If mass of earth were to go up 2x, what would happen to orbital period? Go up Go down Stay the same Can't tell Which one of Kepler's laws applies to Comets? 1 2 3 All of them When you put on a car's brakes, which one of Newton's laws do you feel? 1 2 3 All of them? Which control in a car controls acceleration? 1 Brakes 2 Gas pedal 3 Steering wheel 4 Windshield wipers 5 1-3 Space Shuttle astronauts are... Always falling toward earth Immune to gravity Comet Hyakutake has an orbital period of 65,000 years Comet Hale-Bopp has an orbital period of 4,000 years Which one is heavier? Which one has a larger average distance to the sun? Which one takes longer to orbit the sun? Which one has a larger perihelion (ie, gets closer to the sun)? What determines their orbital period? Their mass (weight) Their maximum distance from the sun (apehelion) Their minimum distance from the sun (perihelion) Their average distance from the sun (semimajor axis) Start talking about Geology --------------------------- Slideshow Four inner planets: Mercury, Venus, Earth, Mars Three moons : Moon, Phobos, Deimos Four major processes: tectonism, Impact cratering, Volcanism, Erosion