Why do things fall—and why do some keep missing the ground?
What Is Gravity?
Gravity is a force of attraction between objects that have mass. Every single object in the universe — from a grain of sand to a galaxy — exerts a gravitational pull on everything else.
The more massive an object, and the closer it is, the stronger its gravitational pull. This is why Earth's gravity holds you to the ground, why the Moon orbits Earth, and why Earth orbits the Sun.
Simple way to think about it: Gravity is like an invisible elastic band connecting every object in the universe to every other object. The heavier and closer two objects are, the stronger the band.
Newton and the Apple
In 1687, Isaac Newton published his Law of Universal Gravitation — one of the most important scientific ideas ever written down. The story goes that an apple fell from a tree near his garden and he began to wonder: if gravity pulls the apple to the ground, does it also pull on the Moon?
Newton worked out that the gravitational force between two objects depends on:
- The mass of both objects (heavier = stronger pull)
- The distance between them (closer = stronger pull, but the force weakens with the square of the distance)
His formula, $$F = G \frac{m_1 m_2}{r^2}$$, describes gravity with extraordinary accuracy and was the dominant model for over 200 years.
Double the mass
The gravitational force doubles.
Double the distance
The force becomes four times weaker.
Triple the distance
The force becomes nine times weaker.
Einstein's Twist
Newton's model was brilliant — but Albert Einstein showed in 1915 that it was incomplete. In his General Theory of Relativity, Einstein proposed that gravity is not a force at all in the traditional sense. Instead, massive objects bend the fabric of space and time — what Einstein called "spacetime."
Imagine placing a heavy bowling ball on a stretched rubber sheet. It creates a depression. If you then roll a marble nearby, it curves toward the bowling ball — not because of a direct pull, but because it is following the curved surface. This is how Einstein described gravity.
Why this matters: Einstein's model predicts things Newton's cannot — like light bending around the Sun (confirmed in 1919), gravitational time dilation (GPS satellites must account for this), and gravitational waves (detected for the first time in 2015).
For everyday purposes — launching rockets, calculating orbits — Newton's equations are accurate enough and much simpler. Einstein's version is needed for extreme situations: near black holes, or when precision is critical.
How Orbits Work
An orbit is not a static position — it is a constant state of falling combined with constant forward motion. This is one of the most counterintuitive ideas in all of physics.
When an object orbits another, it is continuously falling toward it due to gravity. But it is also moving sideways fast enough that as it falls, the ground (or the planet) curves away at the same rate. The result: the falling object never actually hits what it is falling toward.
The classic thought experiment (Newton): Imagine firing a cannonball horizontally from a very high mountain. A slow cannonball falls and hits the ground. A faster one travels further before hitting. Keep increasing the speed and eventually the cannonball travels so fast that as it falls, Earth's surface curves away at the same rate — and it orbits forever. This is exactly what satellites do.
Different orbital speeds produce different orbital shapes:
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Circular orbit: The object moves at exactly the right speed to maintain a constant distance from what it is orbiting.
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Elliptical orbit: The most common type. The object moves closer and further from the body it orbits. Earth's orbit is slightly elliptical — we are about 5 million km closer to the Sun in January than in July.
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Escape trajectory: If an object moves fast enough (Earth's escape velocity is 11.2 km/s), it escapes gravity entirely.
Why Does the Moon Stay Up?
The Moon is falling toward Earth right now. It has been falling for 4.5 billion years. But it never hits because it is also moving sideways at about 1 km per second — fast enough that as it falls, Earth curves away beneath it.
The Moon is gradually moving away from Earth at about 3.8 cm per year. This is because the tidal interaction between Earth and Moon is slowly transferring energy. Eventually — in billions of years — the Moon will be far enough away that it no longer affects Earth's tides the same way. But for all practical purposes, it will remain in orbit indefinitely.
The Moon's orbit in numbers
384,400 km
Average distance from Earth
1.022 km/s
Orbital speed
27.3 days
Time for one full orbit
Gravity and Tides
The Moon's gravity does not just keep the Moon in orbit — it stretches Earth itself. The side of Earth facing the Moon is pulled more strongly than the far side. This difference in gravitational pull creates a slight bulge of water on both the near side and far side of Earth. These bulges are what we experience as tides.
As Earth rotates, different coastlines pass through these bulges — creating the daily rhythm of high and low tides. The Sun also affects tides (though with less force than the Moon). When the Sun, Earth, and Moon align during a new or full moon, their gravitational forces combine to create especially strong "spring tides."
Tides have been critical to life on Earth — they helped create the conditions in tidal pools where early complex life may have developed, and they have been used by humans for navigation and fishing for thousands of years.
Orbit explained simply
Why things stay in orbit
An orbit happens when an object is pulled by gravity but is also moving sideways. Imagine throwing a ball forward. It falls to the ground because Earth pulls it down. Now imagine throwing it so fast that, as it falls, Earth curves away beneath it. The ball keeps falling, but it keeps missing the ground. That is the simple idea behind orbit.
The Moon orbits Earth because Earth’s gravity pulls it inward while the Moon keeps moving sideways. Earth orbits the Sun for the same reason: the Sun’s gravity pulls Earth inward, but Earth’s sideways motion keeps it from dropping straight into the Sun. Satellites work the same way. They are not floating because there is no gravity. They are falling around Earth.
This is why astronauts in orbit feel weightless. They are not outside gravity. They and their spacecraft are falling together, so nothing inside feels like it is being pushed against the floor.
Go deeper
Orbital motion powers services people use every day
Gravity and motion work together. Gravity pulls objects toward each other, while motion keeps objects moving forward. If Earth had no sideways motion, gravity would pull it toward the Sun. If there were no gravity, Earth would travel away in a straight line. The orbit happens because both effects are present at the same time.
This is why satellites can stay above Earth. They are falling toward Earth because of gravity, but they are also moving forward fast enough to keep missing the ground. That continuous falling-around is what we call an orbit.
Why this matters: Weather forecasts, satellite television, navigation, communications and Earth monitoring all depend on choosing and maintaining the right orbit.