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.