The universe is governed by four fundamental forces that dictate how every object, from a tiny atom to a massive star, behaves.

1. Gravitational Force

Gravity is the force of attraction between two objects that have mass. It is the weakest of all forces, but it acts over huge distances. Sir Isaac Newton first described gravity, noting that the force is proportional to the product of two masses and inversely proportional to the square of the distance between them. For example, gravity keeps the Earth orbiting the Sun at an average distance of 150 million kilometers.

2. Electromagnetic Force

This force acts between electrically charged particles. Like charges repel each other, while opposite charges attract. Unlike gravity, this force is incredibly strong. It is responsible for the structure of atoms, holding electrons in orbit around the positively charged nucleus. Without electromagnetic force, chemical reactions and the solid state of matter would not exist.

3. Strong Nuclear Force

Inside the tiny nucleus of an atom, protons are packed tightly together. Since protons are all positively charged, they want to push each other away due to electromagnetic force. The strong nuclear force is the "glue" that overcomes this repulsion to hold protons and neutrons together. It is the strongest force in nature, but it only works over very tiny distances, about $10^{-15}$ meters.

4. Weak Nuclear Force

The weak nuclear force is responsible for certain types of radioactive decay, such as beta decay. In this process, a neutron inside an atom changes into a proton, releasing an electron and a neutrino. While it is much weaker than the strong or electromagnetic force, it plays a vital role in the energy production of stars like our Sun.

• Gravity: Long-range, acts on all mass.
• Electromagnetic: Acts on charged particles, holds atoms together.
• Strong Nuclear: Holds the nucleus together, very short-range.
• Weak Nuclear: Causes radioactive decay, subatomic changes.