Imagine that gravity stopped. This is a thought experiment only; gravity is not something anyone can switch off. But one real law of motion shows what gravity is doing all the time. A moving object keeps moving in a straight line unless something pushes or pulls it, so anything riding on the spinning Earth is held to a curved path by gravity. Earth's surface at the equator moves about 465 meters per second as Earth spins, far slower than the roughly 11,186 meters per second needed to escape Earth. Gravity is what holds anything riding on the spinning Earth to a curved path, and without it that holding would not happen.
Isaac Newton pictured this with a cannon. Newton imagined a cannon on a very tall mountain: fire the ball faster and it lands farther away, and at the right speed it falls around Earth forever, which is an orbit. Everything in orbit is falling around its planet: it moves sideways so fast that it keeps missing the ground. Things on the space station travel at the orbital speed of the station, 17,500 miles per hour (28,000 kilometers per hour). The International Space Station circles the planet once in about 90 minutes. Earth itself moves around the Sun at an average speed of about 29.8 kilometers per second, about 107,000 kilometers per hour.
The Moon is the best-known case. The Moon's average distance from Earth is about 384,400 kilometers, and it takes about 27 Earth days to circle Earth. The Moon has about 1.2 percent of Earth's mass, and its surface gravity is about 0.165 of Earth's. How do we know Earth's pull reaches that far? The Moon is about 60 Earth radii away, so by the inverse-square law Earth's pull there should be 60 squared, or 3,600 times weaker than at the surface. Measured, the Moon accelerates toward Earth at about 0.00272 meters per second squared, against 9.8 for a falling apple, which fits the 3,600 times weaker prediction.
Why does the pull weaken that way? The inverse-square law comes from geometry: gravity spreads thinner and thinner, over an area that grows as the square of the distance from the source. Gravity works the same everywhere in the cosmos; how strongly we feel it depends on how much mass is around. The Sun has about 332,900 times Earth's mass.
Escaping gravity takes speed. Earth's escape velocity, the speed needed to leave it for good, is about 11.2 kilometers per second. The Moon's escape velocity is only 2.38 kilometers per second, about a fifth of Earth's. Gravity is also what holds an atmosphere on. Earth's atmosphere is only about 1/1,200,000 the mass of Earth, a thin skin held on by gravity. Mars, with less mass and weaker gravity, lost most of its original atmosphere. Without a strong enough pull, a world cannot keep its air.
Weight is a different thing from mass. Your weight is how hard gravity pulls on you, and the pull comes from all of Earth's mass. Your mass, the amount of stuff you are made of, stays the same even when your weight changes. On the Moon you would weigh about one-sixth of what you weigh on Earth, because the Moon pulls less. What we call weight in daily life is the feeling of being supported by the ground or a seat pushing back on us. Near Earth's surface, gravity alone makes things speed up at 9.8 meters per second every second, called 1g.
That idea explains the space station. Anything in free fall, like a dropped apple or a falling spacecraft, feels weightless because nothing is pushing back on it. You may have heard that astronauts float because there is no gravity in space. In fact, gravity does not stop in space; astronauts float because they are in free fall, not because there is no gravity. Scientists call the feeling on the station microgravity, about one millionth of normal gravity, rather than zero gravity. There is no such thing as antigravity: no machine can neutralize gravity, though a magnet can balance its pull on one object.
The same pull also holds the Sun together. Inside the Sun, the inward pull of gravity is exactly balanced at each point by the outward push of gas pressure, called hydrostatic equilibrium. The Sun has put out roughly the same energy for billions of years, so its gravity has been balanced by pressure for a very long time.
Words to know
- orbit: curved path around
- microgravity: tiny gravity
- equilibrium: a balance
- velocity: speed and direction
