  We have studied the fluctuation (noise) in the position of sliding blocks
under constant driving forces on different substrate surfaces. The experimental
data are complemented by simulations using a simple spring-block model where
the asperity contact regions are modeled by miniblocks connected to the big
block by viscoelastic springs. The miniblocks experience forces that fluctuate
randomly with the lateral position, simulating the interaction between
asperities on the block and the substrate. The theoretical model provides
displacement power spectra that agree well with the experimental results.
