sentences: a step is a mean of preposition & a block is a kind of object ->  a block and a ramp are kinds of objects  a block and a ramp are kinds of objects & sliding is a kind of moving & a block is sliding down a ramp & friction occurs when two object 's surfaces move against each other -> slip occurs between the unblock and the step friction occurs between the block and the ramp  slip occurs between the unblock and the step & friction causes the speed of an object to decrease & a block is a kind of object -> friction causes the speed of the block to decrease