pool balls are moving on the surface of the felt of the pool table & a pool table is a kind of object & a pool ball is a kind of object & friction occurs when two object 's surfaces move against each other ->  friction will occur between the pool ball and the felt of the pool table  friction will occur between the pool ball and the felt of the pool table & friction causes the speed of an object to decrease & a pool ball is a kind of object -> friction will cause the speed of the pool balls to decrease