title: 2nd Order ODEs in Julia using DifferentialEquations
body: <p>I'm trying to solve the harmonic oscillator using DifferentialEquations in Julia. i.e.:</p>
  
  <pre><code>using DifferentialEquations
  using Plots
  
  m = 1.0                          
  ω = 1.0                     
  
  function mass_system!(ddu,du,u,p,t)
      # a(t) = (1/m) w^2 x 
      ddu[1] = (1/m)*(ω^2)*u[1]
  end
  
  v0 = 0.0                     
  u0 = 1.0                  
  tspan = (0.0,10.0)               
  
  prob = SecondOrderODEProblem{isinplace}(mass_system!,v0,u0,tspan,callback=CallbackSet())
  sol = solve(prob)
  </code></pre>
  
  <p>But it doesn't seem to understand the ODE constructor. Upon running, I get:</p>
  
  <pre><code>ERROR: LoadError: TypeError: non-boolean (typeof(isinplace)) used in boolean context
  Stacktrace:
   [1] #_#219(::Base.Iterators.Pairs{Symbol,CallbackSet{Tuple{},Tuple{}},Tuple{Symbol},NamedTuple{(:callback,),Tuple{CallbackSet{Tuple{},Tuple{}}}}}, ::Type{SecondOrderODEProblem{DiffEqBas
  e.isinplace}}, ::Function, ::Float64, ::Float64, ::Tuple{Float64,Float64}, ::DiffEqBase.NullParameters) at /Users/brandonmanley/.julia/packages/DiffEqBase/avuk1/src/problems/ode_problems
  .jl:144
   [2] Type at ./none:0 [inlined] (repeats 2 times)
   [3] top-level scope at /Users/brandonmanley/Desktop/nBody/nBodyNN/test.jl:25
   [4] include at ./boot.jl:328 [inlined]
   [5] include_relative(::Module, ::String) at ./loading.jl:1105
   [6] include(::Module, ::String) at ./Base.jl:31
   [7] exec_options(::Base.JLOptions) at ./client.jl:287
   [8] _start() at ./client.jl:460
  </code></pre>
  
  <p>Any ideas?</p>