Lecture #7 -- Forces of Nature

I. Forces - definition, how to measure (spring) - unification of falling objects and orbits of celestial bodies - central forces and circular motion II. Orbits - bowling-ball picture-- fast enough to lift slightly off the track - faster still: elliptical orbit - faster still: escape to infinity III. Mechanics - concepts of speed and acceleration - Newton's laws 1) no change in speed or direction if no force 2) forces come in equal and opposite pairs (action/reaction) 3) F = m a IV. 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