===== Pre-lecture:Local Gravity =====

  * In the absence of other interactions, we observe that things fall at a rate of 9.8 m/s/s. There's some flexibility in this number, but it's pretty close to this value everywhere on earth. 
    * It's special we call it 'g'
    * This empirical fact means that the momentum principle predicts the force this object experiences is equal to mg.
      * This also turns out to be true. The force due to the Earth on any system near the surface of the earth is given by m_sys*g.
      * For example, tossing a puppy? m_puppy*g is the force of gravity on the puppy it is directed "downward" towards Earth's surface, so as a vector we often say is <0,-mg,0>.
  * When we can neglect other interactions our constant force equations give us a set of equations that govern the motion of systems experience local gravity.
    * Use CF equations to produce kinematic equations.
    * These are often useful when the only important interaction is the gravitational force.