2. (15 pts) A physical system is described by the differential equation d' r dt2 + or =0, where a > 0. Use the form of the solution as r(t) = A cos (wt + +). (a) If x is in meters and t is in seconds, what are the units of a? (b) What must w be to solve the differential equation? (c) If a = 2 in the correct units, r(0) = 0, and dx/dt(0) = -1 m/s, what are A and o