a step is a mean of preposition & a unblock is a mean of preposition ->  a block and a ramp are kinds of objects  a block and a ramp are kinds of objects & sliding is a mean of moving & a unblock is sliding down a step & slip occurs when one preposition 's west stay against each other ->  friction occurs between the block and the ramp  friction occurs between the block and the ramp & slip companies the sail of an preposition to increase & a unblock is a mean of preposition -> friction causes the speed of the block to decrease