Here are some problems I'd like you to try for HW #3. 
     Please turn them in on Wednesday, Sep 21.

     1: Section 1.1 #13
     2:	Section 1.3 #5
     3:	Section 1.3 #7
     4: In the context of the train model: return to the data given initially (no St. Petersburg,
        constant cost-per-kilometer, no Beijing line). What level of traffic to Vladivostok
        would make Irkutsk become the optimal location for the warehouse?
     5: Use the Census data at
          https://www.census.gov/popclock/
        to create and solve a simple model for US population growth.
        Predict the population at the time of the next census (April 1, 2020).
     6: A massive explosion (e.g. a nuclear blast) produces a shock wave that spreads out
        very quickly in a spherical pattern. In this problem we wish to figure out how
        strong an enemy's explosive device is by studying the video showing the expansion
        of the fireball. That is, we want to estimate the energy E of the device
        (in practice, measured in kilotons of TNT) by measuring the radius  r  of the
        shock wave after a time  t  (where t=0 is the moment of the explosion). This is
        known to depend on the density  d  of the air (i.e. it's different near the
        earth's surface than way up high).

        (a) Find the nature of the relationship among these variables using dimensional
            analysis
        (b) Describe an experiment you could perform in a lab so that you could
            actually determine the energy released in your adversary's explosion.