Here's why:
* Gravitational force is the weakest force but acts over the longest distances. It's responsible for the attraction between objects with mass. While it's weak on the scale of everyday objects, it's the dominant force on astronomical scales, holding galaxies together.
* Electromagnetic force is much stronger than gravity but acts over shorter distances. It governs the interactions between charged particles and is responsible for phenomena like electricity, magnetism, and light.
* Strong nuclear force is the strongest force in nature but acts only over very short distances, holding protons and neutrons together in the nucleus of an atom.
* Weak nuclear force is responsible for radioactive decay and is weaker than the strong nuclear force.
While the strong nuclear force is the strongest overall, its strength is concentrated over very short distances. When considering force per unit mass, the gravitational force is the weakest.
