Question

Why are gravitational, electromagnetic, and nuclear forces often called fundamental or basic forces?

Answer 1

It's because everything we perceive as force can be rendered as an electromagnetic, gravitational, or nuclear force, or some combination of these.

Explanation:

Let us work from a concrete example. Suppose you are pushing a load across the floor. As you do so you feel your hands pressing against the load.

Now think about what is happening at the atomic level. As you press your hand against the load the atoms of your hand and the load are closer together than the preferred separation they they would assume if no force were acting. Because of the way negatively charged electrons and positively charged nuclei are arranged in atoms, pressing them too close together creates electromagnetic forces that cause the load to be repelled from your hand. You think the load moves ahead of you because of your strong muscles, but a physicist tells you it's because of the electromagnetic force between atoms.

As a more complex example, consider a cork floating on water. The atoms in the cork and the atoms in the water push each other around through the electromagnetic force as described above because of the incompressibility (at low pressure) of the cork and water. So gravity can pull the cork down into the water only by allowing water to go up around the cork ... against gravity. We are in effect forcing gravity to make a choice, what we call buoyancy.

Thus the equilibrium state of the cork floating on the surface of the water comes from balancing gravitational forces on the cork and the water, with the balance constrained also by the electromagnetic forces between the atoms of the two objects. Both electromagnetic and gravitational forces are involved in making the cork float.

And so it goes with any phtsical process. It is harder to see examples involving the nuclear forces, because unlike the other two types of fundamental force these have only a short range and are confined to atomic nuclei. Yet physical processes in those nuclei, including the formation of the nucleus and radioactive decay, can be resolved into strong and weak nuclear forces, just like the above examples being resolved into electromagnetic and gravitational forces.

Socratic says this is getting long, so I am done. The moral of the story: any force we can think of can be rendered in terms of electromagnetic, gravitational, and strong or weak nuclear forces.