The "Force" Seen Across Different Scales
Have you ever looked at a moving object and wondered why it moves the way it does? Tree branches swaying in the wind, a bouncing ball, fish swimming in water—behind all of these, invisible "forces" are at work.
The ancient Greek philosopher Empedocles believed that the world was bound together and torn apart by two forces: "Love (Philia)" and "Strife (Neikos)." While this is an extremely abstract way of thinking, it can be considered one of the earliest ideas to capture the intuition that distant objects influence each other as a "force."
In the 17th century, Newton discovered universal laws governing motion. Newton's equations of motion, which state that force acts on an object as acceleration, can explain everything from the movement of planets to the falling of an apple using the same principle.
Later, the perspective of physics expanded from classical mechanics to an even smaller world, moving into quantum mechanics. Even in the quantum world, electromagnetic forces act between the nucleus (+) and electrons (-). Furthermore, the reason an atomic nucleus consisting of protons and neutrons remains intact without collapsing is that there is a force that acts strongly over short distances, overcoming electrical repulsion to bind the nucleons together.
As the scale increases, atoms and molecules can be treated as particles. In molecular dynamics, the motion and structure of molecules are reproduced by assuming the forces acting between each atom and evolving Newton's equations of motion over time. Forces acting between molecules are generally expressed as van der Waals interactions or Coulomb interactions, but as the type and size of the molecules change, the interactions between the resulting particles take on different forms. In other words, depending on the scale being considered, the forces acting between the objects of focus—or the "manifestation of force at that scale"—change.
From an even more coarse-grained perspective, particles appear to move while being randomly shaken and subjected to friction from their surroundings. In this case, the effects of fine collisions are collectively expressed as "friction" and "fluctuation." By averaging out fine degrees of freedom, only the essential interactions remain, revealing why different substances follow the same laws. Forces once intuited as love or gravity are now understood in multiple layers through the interplay of theory and measurement. Correctly understanding the forces acting on molecules and particles leads not only to the elucidation of new phenomena but also to the development of various "things" and technologies that support our lives.