Real Oscillators … constant forces integrate EOM parabolic trajectories. … linear restoring force guess EOM solution SHM … nonlinear restoring forces ? linear spring nonlinear spring? F F x x
WTF! The spring of air : use Ideal Gas Law: PV=NRT Patm chamber volume: V=Ax WTF! (whoa there, fella) m EOM A +x Stable Equilibrium at xeq = NRT / (mg + APatm) P, V
Taylor Series Expansions: Turns a function into a polynomial near x = a Example:
Expand around x = -3: 2nd order 0th order 1st order
Expand around x = 2: 0th order 1st order 2nd order
Expand NRT/x around xeq: Is it safe to linearize it? Better check a unitless ratio. How about: (Yes, excellent choice Dr. Hafner!)
.. Displacement 5% of xeq: 0 .05 .0025 …. Perhaps you would prefer…. SHM with
Beware of assumptions and approximations!
Clerk = person who derived an approximation Clouseau = You Clerk = person who derived an approximation Dog = domain of interest Clerk’s dog = domain of his approximation Bite = result of mistaken assumption that you are working within the Clerk’s domain
Simple Pendulum: Stable Equilibrium: Length: L Mass: m Q Displace by Q: mg cosQ T mg cosQ sinQ -x mg cosQ EOM: mg Expand it! mg
Derivatives:
Now express as a unitless ratio of the dependent variable and some parameter of the system: Displacement 5% of length: 0 .05 0 .0000625 … SHM with
The world is not linear. However, one can use a Taylor expansion to linearize an EOM by assuming only small perturbations around a point of stable equilibrium (which may not be the origin).