We consider here the situation of a stationary particle acted on by a number of forces. It is not very useful to attempt to define exactly what we mean by a force: examples of forces will suffice. But we can think of a force as something that tends to produce motion. A force is therefore obviously a vector quantity. There are (as far as is known at present) four fundamental forces: gravity, electromagnetism, Weak nuclear force and strong nuclear force. Each force is accompanied by a theory and a set of equations governing the behavior of the force and objects affected by the force. All other forces are derived from these forces. Examples in no particular order are friction, tension in strings, normal reaction forces, air resistance, viscosity, magnetism, gravity, van der Waals forces between molecules, etc. I’m sure that you can think of many more examples. For example, for a particle on a rough horizontal table being pulled by a string (though not hard enough to make the particle move), the forces are as shown in the diagram. There are two external forces, namely the applied (pulling) force acting along the string, and the weight acting downwards. The table exerts two forces on the particle: one is the force of friction which tends to oppose motion; the other is the reaction of the table on the particle that stops the particle falling through the table. This latter force is normal to the surface of the table and is called the normal reaction.