Lecture #8 -- More on Newton and Forces

I. Newton's laws
1) no change in speed or direction if no force
2) forces come in equal and opposite pairs (action/reaction)
3) a = F/m (often written F = m a)
II. Newton's law of gravity
- F = G*M_1*M_2/D^2
- note it is an inverse-square law like light
- note it is equal and opposite for the two masses
- combine with F = m a to derive acceleration of gravity g
- note the acceleration, so the motion, does not depend on the object at all
III. How Newton's laws give you Kepler's main law