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

Let's start with a practical example: Imagine that you are pushing a load across the floor and that your hands are pressing against the load.

The load is being repelled from your hand because of electromagnetic forces created by the atoms' arrangement of positively and negatively charged nuclei. You may believe that the load is moving ahead of you due to your strong muscles, but a physicist explains that the electromagnetic force between atoms is what causes the load to move. Now consider 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 would assume if no force were acting.

For a more complicated example, think about a cork floating on water. Because of the incompressibility (at low pressure) of the cork and water, the atoms in the two substances push against each other through the electromagnetic force as previously described. As a result, gravity can only pull the cork down into the water by allowing the water to rise around the cork—that is, against gravity. In this way, we are effectively forcing gravity to choose, which is known as buoyancy.

The cork floats because of both electromagnetic and gravitational forces acting on it. Specifically, the equilibrium state of the cork floating on the water's surface results from balancing the gravitational forces acting on it and the water, with the balance being constrained also by the electromagnetic forces between the atoms of the two objects.

As is the case with all physical processes, it is more difficult to find examples involving nuclear forces because, in contrast to the other two categories of fundamental forces, they are limited to atomic nuclei. Nevertheless, nuclear processes, such as nucleus formation and radioactive decay, can be separated into strong and weak nuclear forces, just as the electromagnetic and gravitational forces in the previous examples can be separated into nuclear forces.

The moral of the story is that any force we can think of can be expressed in terms of electromagnetic, gravitational, and strong or weak nuclear forces. Socratic says, "This is getting long, so I'm done."

Since they are the fundamental interactions that control the behavior of matter at the most fundamental level, gravitational, electromagnetic, and nuclear forces—which account for all particle interactions in physics and are crucial to comprehending the behavior of matter in the universe—are also referred to as fundamental or basic forces.