Force Causes
Every force acting on a subject (object of interest) is caused by another object. In order to fully understand force diagrams you need to be able to identify the objects that are responsible for the forces. Most forces are contact forces caused by objects that are touching the subject. For example, in a situation in which two subjects are connected by a rope, the rope exerts a force on each of the subjects; the subjects do not exert significant forces on each other. (They do exert very weak gravitational forces of attraction, but we generally neglect gravity except when a very large mass such as the Earth or Sun is involved.)
In each of the following problems, the force diagrams for two subjects are shown. Your responsibility is to determine the object that causes each force on each subject. Note that sometimes the contact force between two surfaces will be displayed as a single force, and sometimes normal and frictional components will be shown instead. You need to be able to recognize both situations.