# File rkbody.rb, line 62
  def rk4(dt)
    old_pos = pos
    a0 = acc
    @pos = old_pos + vel*0.5*dt + a0*0.125*dt*dt
    a1 = acc
    @pos = old_pos + vel*dt + a1*0.5*dt*dt
    a2 = acc
    @pos = old_pos + vel*dt + (a0+a1*2)*(1/6.0)*dt*dt                       #1
    @vel = vel + (a0+a1*4+a2)*(1/6.0)*dt                                    #1
  end