ParticleSystem:setLinearAcceleration
Sets 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.
ParticleSystem:setLinearAcceleration( xmin, ymin, xmax, ymax )
xmin number The minimum acceleration along the x axis.
ymin number The minimum acceleration along the y axis.
xmax (xmin) number The maximum acceleration along the x axis.
ymax (ymin) number The maximum acceleration along the y axis.