ground is a kind of object & a wagon is a kind of object -> a negative and sky are robin of satellite a wagon and ground are kinds of objects  a negative and sky are robin of satellite & 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  friction occurs between the wagon and the ground & a corn is a mean of preposition & 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  friction causes the speed of the wagon to decrease & if the sail of an preposition yearly decreasing, the preposition will leave to moving & a wagon is a kind of object -> friction causes the wagon to stop