Lesson 4: Gravity — The Force That Shapes Everything
Gravity — The Force That Shapes Everything
Gravity is the reason you're sitting in your chair right now, the reason the Moon orbits Earth, and the reason the Earth orbits the Sun. It's the weakest of the fundamental forces — but it has the longest range, which makes it the force that shapes the entire large-scale structure of the universe.
Newton's Law of Universal Gravitation
Every object with mass attracts every other object with mass. The force depends on two things: how massive the objects are, and how far apart they are.
Double the mass of one object → double the gravitational force. Double the distance → the force drops to one quarter. This is called an inverse square law.
Why Do Things Orbit?
An orbit is basically a continuous fall. The Moon is falling toward Earth right now. But it's also moving sideways so fast that by the time it falls a little, Earth's surface curves away underneath it. It keeps falling but never hits.
Newton imagined firing a cannonball from a very tall mountain. Fire it gently and it falls to the ground. Fire it faster and it goes farther before hitting. Fire it fast enough and it falls at the same rate Earth curves away — it orbits.
The International Space Station orbits at about 28,000 km/h. At that speed, the astronauts are in constant freefall — that's why they float.
Escape Velocity
To leave Earth completely you need to travel at 11.2 km/s (about 40,000 km/h). That's escape velocity. The Moon's escape velocity is much lower (2.4 km/s) because it has less mass. That's why the Moon has no atmosphere — gas molecules can escape its weaker gravity.
Einstein's Upgrade: Gravity as Curved Space
Newton described gravity perfectly for most purposes. But Einstein realized something deeper: massive objects don't just pull — they warp the fabric of space and time itself. Other objects follow the curves in that warped space. What we feel as gravity is actually us following the straightest possible path through curved spacetime.
This sounds abstract, but it has real consequences. GPS satellites have to correct for Einstein's relativity. Their clocks tick slightly faster in weaker gravity up in orbit. Without the correction, GPS would drift by about 10 km per day.
Black Holes
When a massive star dies, it can collapse into a point so dense that gravity becomes strong enough to trap even light. Nothing can escape — not matter, not information, not light. The boundary of no return is called the event horizon.
Black holes aren't cosmic vacuum cleaners. If our Sun became a black hole (it won't), Earth would continue orbiting exactly as before — same mass, same gravity. We'd just lose the light and heat.
Think About It
- The Moon causes ocean tides. How? (Hint: the side of Earth closest to the Moon feels slightly stronger gravity than the far side.)
- Why do heavier objects not fall faster than lighter ones?
- What would happen to your weight on a planet twice as massive as Earth but the same size?