At a “macro level”, there appears to be two categories of forces: those that require direct contact to occur, and those that are able to act across distance and empty space.
|Contact Forces||Action-at-a-Distance Forces|
|Normal Contact Force||Gravitational Force|
|Frictional Force||Electrical Force|
|Drag Force||Magnetic Force|
At the “micro level”, you will learn that these so-called contact forces are in fact electrical forces. For example, when you “touch” the wall, the molecules of your fingers are actually not in contact with the molecules of the wall. The so-called normal contact force is actually the action-at-a-distance electrical repulsion between molecules. Nevertheless, in most “everyday” problems, it is helpful to distinguish between contact and non-contact forces.
You’ll be studying the action-at-a-distance forces (also called field forces) in detail in the coming chapters. Meanwhile, you should read up on the following to help you understand the properties of some of the common forces: