The tube of radius r is filled with a porous substance of density ρ0. The piston, which acts on a constant force F, moving in the pipe, compacts the substance to a density ρ. At what speed does the piston move if the compaction of the substance occurs in a jump, i.e. the interface moves at a certain speed in the pipe, to the right of which the density of the substance is ρ0, and to the left - ρ? At the initial moment, this boundary coincides with the surface of the piston.