Some common types of forces that you will be dealing with include the gravitational force (weight), the force of tension, the force of friction, and the normal force.
It is sometimes convenient to classify forces as either contact forces between two objects that are touching or as long-range forces between two objects that are some distance apart. Contact forces include tension, friction, and the normal force. Long-range forces include gravity and electromagnetic forces. Note that such a distinction is useful but not really fundamental: For instance, on a microscopic scale the force of friction is really an electromagnetic force.
In this problem, you will identify the types of forces acting on objects in various situations. First, consider a book resting on a horizontal table.
Part A: Which object exerts a downward force on the book?
the book itself
the earth
the surface of the table.