Note to the teacher:
Provide students with a photograph, magazine picture, or cartoon and have them list (and label if possible) as many forces shown as possible. They are to use their own words. Share with the rest of the class. At the conclusion, let students know that there are many ways to classify and categorize forces, and that they may revise their ideas about the classifications they have chosen once they have completed more study of forces.
Scientists have classified forces into four fundamental categories: gravitational, electromagnetic, weak nuclear, and strong nuclear. Notice that many common forces such as pushes and pulls, tension and spring forces, friction and normal forces are not category labels.
This means that each of these forces is really a manifestation of some fundamental force. For simplification, the electrical nature of these forces will be discussed first, and then the unification of the electrical and magnetic force into the electromagnetic force will be examined.
The normal force arises at the atomic level as a manifestation of the electric force between charged particles within the atoms of the two surfaces in contact. Because the weight of a block is pressing it "into" a table top that it is resting on, for example, the atoms in the table top become compressed beyond their equilibrium positions. The electrical forces between the subatomic particles create "restoring" forces that push upward on the block in an effort to restore equilibrium on the atomic scale.
Tension and spring forces are the result of similar distortions in the equilibrium subatomic arrangements. When tension is applied to a rope, the cohesive electric forces between atoms in the rope cause the force to be transmitted, particle to particle, to the other end of the rope, and to any object to which the rope is attached. Similarly, when springs are stretched or compressed, the subatomic particles exert unbalanced electrical forces on one another to restore the spring to its equilibrium status.
Friction is also an example of an electrical force in that, as two surfaces are in close contact with one another, there are electrical attractions between the subatomic particles in the two surface. Other forces must be applied to "break" these temporary bonds and cause the surfaces to slide across one another.
The MKS system standard unit of measurement of force is the newton, regardless of what type of force is being measured. In the case of common forces such as friction, any measurement of force is actually the measurement of the cumulative effects of millions of electrical forces acting concurrently.
Experiments have shown that the electrical and magnetic force are essentially the same fundamental entity, so have been categorically combined into the "electromagnetic" force. Electromagnetic phenomena such as the operation of generators and motors are commonplace, but the electromagnetic nature of television is more subtle. The facts that electric currents produce magnetic fields, and that magnets can exert forces on beams of moving electrons confirm the connection between electrical and magnetic forces.