Forces continued...

A force can simply be defined as a push or pull. There are different types of forces that include contact forces, net forces, frictional forces, etc.  Contact forces are forces in which at least two objects are touching.  A net force is a force that is unbalanced.  Balanced forces have no acceleration and a constant velocity.  Frictional forces are forces that oppose motion or impending motion. I took a picture of a stack of books.  If a free body diagram of the books were to be drawn it would show that they are both affected by 2 forces.  The book on the bottom is affected by weight down (mg), and the normal force up (N).  The book on the top is affected by weight down (mg), and the force of the textbook below it (F).