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PHY134 Introductory Astronomy

PHY134 Introductory Astronomy . Newton and Gravity. What Newton Told Us. . The rest is details If we can figure out forces this is a way to predict from where things are today where they will be in future (or were in past ):. positions, velocities. forces.

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PHY134 Introductory Astronomy

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  1. PHY134Introductory Astronomy Newton and Gravity

  2. What Newton Told Us • . The rest is details • If we can figure out forces this is a way to predict from where things are today where they will be in future (or were in past): positions, velocities forces accelerations advance t

  3. Conservation Laws • Mathematical theorems follow from Newton • Momentum then is rate of change of • So if A and B act on each other, does not change – they exchange momentum but can’t create or destroy it. Momentum conserved

  4. More Conservation • With a little more math, see that a circular version – angular momentum is also conserved • This will be incredibly important to us – things in space spin • Earth spins at constant rate because it once did

  5. Energy • If gravity is the only force acting on an object, can show that total energy is constant • In general, other forces act. Find that this introduces more kinds of energy: sound, light, heat, chemical, electric, nuclear, etc. but total is conserved • Units of energy:

  6. Attractive Logic • If an object of mass moves in a circle of radius with uniform speed there must be a force acting, directed to center, of magnitude • Moon orbits Earth so force directed towards Earth • We notice Earth applies such a force to apples. Could this be the same force? • Planets orbit Sun in (almost) circular orbits at (almost) uniform speed. Does Sun apply the force this implies on all planets? • If so, Earth must apply a force to Sun directed towards Earth • It all hangs together!!

  7. Kepler and Newton • Planet of mass orbits Sun at radius with speed • Force Sun applies to planet is thus • Kepler says • Find

  8. It’s Universal • Sun applies a force given by to each planet, with same • So each planet applies force of same magnitude to Sun • Law does not single out planet from Sun, so must have so

  9. Really Universal • Earth also attracts the Moon, and all objects near it. • So Moon and all other objects attract Earth. • Everything attracts everything else! • Measured Newton’s constant:

  10. Thinking about Orbits • Why doesn’t the Moon fall on Earth? • It does! Moon is constantly accelerating towards Earth. Orbiting is falling without ever hitting the ground

  11. Success! • Newton’s equations predict Kepler’slaws. • Also show there are other types of orbits. • New successes: Halleypredicts return of comet (1757). • Neptune discovered mathematically in details of Uranus’s orbital motion.

  12. Even More Generally • Newton’s law is Universal. Apply to any two objects orbiting under mutual gravity. Find elliptical orbit about center of mass with

  13. Example: Low-Earth Orbit • ISS orbits at an altitude so has orbital radius • Period given by

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