When you drop a book, there must be a force on it because it does not continue where you released it. We recognize this as the force of gravity and will talk much more about it later. But what happens when you place a book on a table and it just sits there? Has this force of gravity suddenly disappeared? No, the table is supporting the book. The force the table exerts is very real and very important -- but it is often easy to overlook such forces of support! Watch out for them.