A block of mass $M$ slides on a frictionless horizontal track, pushed by a constant horizontal force $F$. Spaced every distance $d$ along the track are stationary small blocks, each of mass $m$. Every collision between the moving block (and any debris already stuck to it) and the next small block is perfectly inelastic. After absorbing a very large number of small blocks, the speed of the system just before each collision approaches a limiting value. What is this limit?