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Lecture 9

# Lecture 9

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School
Department
Physics
Course
PHYS 183
Professor
Tracy Webb
Semester
Winter

Description
PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013 th Lecture 9: January 25 , 2013  a law is a description of the way the universe behaves  the universal laws of gravitation o every mass attracts every other mass  the earth is attracting you to the earth, but you are also attracting the earth o the strength of attraction is directly proportional to the product of their masses  if you double the mass of something, the force of gravity is doubled too o the strength of attraction is inversely proportional to the square of their separation distance  if distance goes up, the force of gravity goes down (not linear) 2 o Fg= G (M1M 2d ) o force of gravity = gravitational constant (mass of object 1 x mass of object 2 / distance between the two objects squared)  usually rare a scientific theory is completely wrong  as we gain new knowledge we usually change - the theories, not start from scratch  centre of mass o objects orbit around their centre of mass o for 2 objects of equal mass, centre of mass lies halfway between them o for 2 objects of different masses, centre of mass lies closer to the more massive one o if 1 object is so much more massive that the planet, the centre of mass can lie inside the star o important for extra solar planets  Newton and Kepler’s Third Law o Kepler found P = a for our solar system (P in years, a in AU) o Newton showed that this is a special case of the general behaviour of orbits o using the orbital period and orbital distance we can now find the total mass of a system  measuring masses o for the special case of one dominant mass (and a small satellite) o the earth’s orbital period (1 year) and its distance from the sun (1 AU) tells us the mass of the sun o the orbital period & distance of a satellite orbiting the earth tells us the earth’s mass o the orbital period and distance from Jupiter of one of Jupiter’s moons tells us the mass of Jupiter  orbits, tides and acceleration
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