hook
Imagine sitting under a tree on a sunny day, and suddenly - plunk! - an apple falls right beside you. Annoying, right? But in 1666, a similar moment sparked a scientific revolution. Isaac Newton looked at that falling fruit and asked: "Why did it fall DOWN?"
phenomenon
Gravity - the invisible force that pulls objects toward each other. It’s why apples fall, why we don’t float off into space, and why planets orbit stars. Newton didn’t discover gravity (people knew things fell!), but he was the first to explain it with math.
explanation
Newton’s big idea was simple: Every object with mass attracts every other object. The more massive the object, the stronger the pull. And the farther apart they are, the weaker the pull.
He captured this in his Law of Universal Gravitation:
F = G * (m1 * m2) / r²
Where:
- F = gravitational force (in Newtons, N)
- G = gravitational constant (6.674×10⁻¹¹ N·m²/kg²)
- m1, m2 = masses of the two objects (in kilograms, kg)
- r = distance between their centers (in meters, m)
Key insight: Gravity gets weaker with the square of the distance. Double the distance → force is ¼ as strong. Triple the distance → force is 1/9 as strong!
visualization
Let’s picture Earth and an apple:
EARTH (m1 = 5.97×10²⁴ kg)
/ |
/ | r = 0.5 m (distance from center)
/ |
/ | F = G * (m1 * m2) / r²
/ |
/_______|
APPLE (m2 = 0.1 kg)
Even a small mass like an apple (0.1 kg) feels a noticeable pull from Earth’s huge mass (5.97×10²⁴ kg)!
application
Why does this 300-year-old idea matter today?
- Space Travel: We use gravity to slingshot spacecraft around planets (saving fuel!).
- GPS Satellites: They must account for Earth’s gravity to give accurate positions.
- Tides: The Moon’s gravity pulls on Earth’s oceans, creating tides.
- Your Weight: Your weight is just the gravitational force between you and Earth!
think_about_it
🤔 If you could turn off gravity for 1 minute, what would you do? Would you float? Would the air disappear? How would Earth itself react? Newton’s law shows gravity isn’t just about falling apples—it’s the glue holding our universe together!
Historical Context:
- Isaac Newton (1643-1727)
- Year: ~1666 (during the Great Plague)
- Breakthrough: First mathematical description of gravity
- Legacy: Foundation for classical physics
- Fun Fact: The apple story might be a myth, but the math is very real!
Difficulty: (Beginner)
Physics Explorer Series: Where every equation tells a story.