Definition
Gravity is the universal attraction between all masses. In classical physics, it is described by Newton’s Law of Universal Gravitation:
- How to read: “The force F is equal to G times the product of m one and m two, all divided by d squared.”
- Meaning: Gravitational force is proportional to both masses and inversely proportional to the square of distance — Newton’s inverse-square law.
Why It Matters
Gravity is the fundamental ‘architect’ that binds the cosmos; it is the force that shapes planets, stabilizes orbits, and ensures that the universe remains a structured system of matter rather than a chaotic cloud of atoms.
Core Concepts
- Newtonian Synthesis: The realization that the same laws of motion and gravity apply to both terrestrial objects (falling apples) and celestial bodies (the Moon).
- Inverse-Square Law: The force of gravity decreases with the square of the distance. Doubling the distance reduces the force to .
- Weight vs. Weightlessness:
- Weight: The force an object exerts against a supporting surface (e.g., a scale).
- Weightlessness: A condition where there is no support force, such as during free fall. Orbiting astronauts are weightless because they are in a state of continual free fall around the Earth.
- The Weakest Force: Gravity is the weakest of the four fundamental forces, but its range is infinite and it is always attractive.