Gravity: Mass Attracting Mass

1. From Falling Apples to Universal Attraction

Local observation
Everyday experience tells us that objects near Earth “fall” toward its center. Galileo’s inclined-plane experiments showed that this motion is uniform (after correcting for friction), suggesting a consistent force pulling things downward.

Newton’s great leap
Isaac Newton realized that the same pull causing apples to drop also governs the Moon’s orbit around Earth—and, indeed, the planets’ orbits around the Sun. He postulated a universal force of attraction between any two masses, no matter how small or how far apart.

2. Newton’s Law of Universal Gravitation

Newton encapsulated “mass attracting mass” in a single formula:

• 𝐹 is the magnitude of the gravitational force between two point-masses.
• 𝑚₁, 𝑚₂ are their masses.
• 𝑟 is the distance separating them.
• 𝐺 is the gravitational constant, setting the strength of the interaction.

Because 𝐹 scales with the product of the two masses, and pulls along the line joining them, this law precisely captures “mass attracts mass.”

3. Why “Attraction” Proves Gravity

• Mutual Interaction
  If mass A pulls on mass B, then by Newton’s third law, B pulls equally on A. This reciprocity is the hallmark of a genuine force, not a one-sided “influence.”
• Inverse-Square Dependence
  The fact that the force weakens with the square of distance matches both the geometry of space (flux spreading over spheres) and the observed motions of planets and comets.
• Measurable Constant 𝐺
  Henry Cavendish’s late-18th-century torsion-balance experiments quantified 𝐺, demonstrating that even tiny lead spheres exert measurable attraction. This cemented that gravity isn’t just a metaphor—it’s a real force, with a precise numerical strength.

4. The Foundation of the Idea

• Universality: Unlike friction or electromagnetism, gravity acts on all masses alike, regardless of composition.
• Predictive power: With “mass attracts mass” as its axiom, Newtonian gravity can predict tides, planetary orbits, and spacecraft trajectories to astonishing precision.
• Gateway to deeper theories: Einstein’s general relativity replaces the “force” picture with curved spacetime, but still reduces to “mass attracts mass” in everyday situations. In other words, even in modern physics the statement remains the bedrock.

5. In a Nutshell

“Mass attracting mass” isn’t just a poetic slogan — it is the operational definition of gravity. From falling apples to orbiting moons, from Cavendish’s laboratory to GPS satellites, every gravitational phenomenon flows from the simple fact that two masses exert a mutual pull on one another.

That mutual pull—and our ability to measure and encapsulate it in a universal law—is exactly why “mass attracts mass” proves the existence of gravity, and why it underlies our entire understanding of gravitational phenomena.