A method to measure the average normal force on a surface produced by inter
mittent-contact (tapping) force microscopy is presented. This force is dete
rmined by measuring the average deflection of a calibrated piezoresistive c
antilever in intermittent contact with an oscillating active cantilever. Re
sults obtained with this method are presented for a two-state cantilever mo
tion where the piezolever force is higher for the state with the lower ampl
itude of vibration. (C) 2000 American Institute of Physics. [S0003-6951(00)
02007-6].