1st - if you slide a book across a table, it will eventually slow to a stop because of the opposite force friction is exerting or it will come to a stop once it hits the wall.
2nd - It's easier to push an empty box than a box thats halfway full because the empty box has less mass. So more force is needed to move the filled box.
3rd - When jumping off a diving board, the springboard is pushed down by your body weight and then it springs back up, pushing you into the air.
