ground is a kind of object & a wagon is a kind of object ->  a wagon and ground are kinds of objects  a wagon and ground are kinds of objects & when the wagon is moving down the hill, it is moving against the ground & friction occurs when two object 's surfaces move against each other ->  friction occurs between the wagon and the ground  friction occurs between the wagon and the ground & a wagon is a kind of object & friction causes the speed of an object to decrease ->  friction causes the speed of the wagon to decrease  friction causes the speed of the wagon to decrease & if the speed of an object keeps decreasing, the object will come to stop & a wagon is a kind of object -> friction causes the wagon to stop