anchor: ParticleSystem:getLinearAcceleration
  Gets the linear acceleration (acceleration along the x and y axes) for particles.
  
  Every particle created will accelerate along the x and y axes between xmin,ymin and xmax,ymax.
  xmin, ymin, xmax, ymax = ParticleSystem:getLinearAcceleration()
  xmin number The minimum acceleration along the x axis.
  ymin number The minimum acceleration along the y axis.
  xmax number The maximum acceleration along the x axis.
  ymax number The maximum acceleration along the y axis.