Lecture 4-5: Galileo, Tycho, Kepler, Newton

1. an imperfect sun (sunspots)
2. a moon with mountains and craters
3. four moons orbiting jupiter
4. the phases of venus

he made extremely accurate measurements of the positions of the stars and planets over the course of 20 years

"geocentric with modifications"
-earth at center (parallax not seen)
-sun travels about the earth in a perfect circle
-the planets move around the sun in perfect circles

1. Planets move in ellipses with the Sun at one focus
2. Planets in their orbits sweep out equal areas in equal times
3. The period P of an orbit (in years) squared = a, the semi major axis of the orbit (in AU) cubed (P squared = a cubed)

1. A body's velocity will remain constant, unless acted upon by an outside force (inertia)
2. A body's acceleration depends on the force acting upon it and will be in the direction of that force
3. for every action, there is an equal and opposite reaction

the force of gravity depends on the masses of the two bodies and their separation squared;
the larger the mass, the greater the attraction;
the larger the separation, the smaller the attraction
F=Gm1m2/r squared

Because of Newton's third law of motion: for every action, there is an equal and opposite reaction)

Gravity is pulling you down but the ground is not allowing you to fall; it must therefore be exerting a force on you-- weight.

Free fall

because the earth was born with INERTIA, it creates a curbed path with gravity around the Sun