Remember, the net force is the sum of all the forces acting on an object. Some of those forces may not be entirely obvious. Some of those forces are easy to overlook or to forget.
Consider pushing a book along a table,
The force you exert on the book is clear and obvious and easy to think of. We might lablel it Fext, meaning, and "external force",
But is that the only force acting on the book? Hmmmmm, . . . No that is not the only force acting on the book. If we pick the book up and drop it, we notice that gravity exerts a force on the book; we call this force the weight of the book and we can label it W. What happens to the book when it touches the table? The table pushes up on the book with a support force. We could label this Fsupport. This support force of the table is also sometimes called a "normal force" and we could also label this as FN.
Do we have all the forces now? No, not yet. There is still a force of friction between the table and the book. This force of friction opposes the motion and acts the the direction opposite to the motion of the book. We can label this force of friction as Ffrict.
Now we have all of the forces. Now we can talk about the net force. Remember, the net force is the sum of all the forces. We can write this as
where , the Greek letter SIGMA, simply means "sum of" and Fi represents each of the individual forces.
Friction is almost always present. It is often easy to overlook or miss. Sometimes we will make an approximation and intentionally leave it out. Be sure that is what you intend to do tho'. Never leave it out just because you forgot it.