be understood that if desired the spring member 24 could be full width for the entire length thereof and the dogs 32 could be adapted for movement away from the spring member 24 as illustrated by the double headed arrows in fig6 to release the spring member and provide the 34 whip 34 action desired for impacting the bottom sheet in the stack one of the significant advantages of the impact sheet feeder with air floatation is that positive control is exercised over the inter sheet frictional forces due to the air lubrication provided between sheets by the air floatation tray therefore the problem of mechanical coupling of the sheets during impact is eliminated specifically this means that the driving force on the second sheet in the stack is very small since the coefficient of friction is greatly reduced on a top feeding impact type device the coefficient of friction between sheets u03bc pp typically may average 0 55 with the device disclosed it has been found that the coefficient of friction is greatly reduced i e u03bc pp may typically average 0 02 0 10 further it has been found that with air floatation the inter sheet frictional coefficients are monotonically increasing i e u03bc pp between sheets 1 and.