A force is an action applied to an object. As a consequence of the force, the object may move, change shape or any other characteristic of the body. Forces are classified accordingly with the following guides:

 Forces Contact Forces Push (including kicks) Pull Non-contact Forces Gravitational Electric Magnetic Nuclear Strong Weak

Most of the forces study in these notes are contact forces.

In the international system of measurement, the unit of force is the Newton (N). The Newton is not a fundamental unit of measurement, it is derived from the meter, kilogram, and second. In fact,

The most important characteristic of forces is that they are vectors. As such, forces have magnitude and direction, ; graphically, forces are represented by arrows. The symbols to represent forces are the same as those to represent any other vectors, .

Algebraic operations between forces follow the rules for vector's operations. In particular, the vector addition rules apply to forces. The following diagram illustrate the basic steps for obtaining the result of applying different forces over an object.

 Forces -- Figure 1 -- Addition of Forces In particular, these figures explain the manifestation of the vector characteristic of forces when apply to an object. The left most part of the diagram represents the actual forces acting on the object including their directions as well as their relative magnitudes. In the middle part of the diagram, the two forces are considered as applied to the same point in the object (see center of mass xxx) because, for simplicity, object are considered punctual even when they are represented by dimensional draws. Until rotations are studied, all objects are in reality considered points in space. The red diagonal vector, F, is the vector addition of the two vectors that represent the two applied forces, F1 and F2. In the right most part of the diagram, the net effect of the two applied forces is the force F pulling the object in the direction pointed by the red arrow. Thus, the object does not move either horizontally or vertically. The object moves diagonal in the direction represented by the vector F.

When several forces act on an object, the vector addition of these forces is called the net force, .

 1)

If the vertical force is 4 N and the horizontal force is 3 N; then the net force is

 a) 7 N b) 1 N c) 25 N d) 5 N e) None of the above.

 by Luis F. Sáez, Ph. D. Comments and Suggestions: LSaez@dallaswinwin.com