Gravity is a force that pulls things toward each other. It can only pull, never push. Earth's gravity keeps you on the ground and makes things fall. It is why things fall down when you drop them or throw them. A dropped thing falls faster and faster as it falls. Galileo showed that, ignoring air resistance, heavy and light things speed up at the same constant rate as they fall. Near Earth's surface, a falling thing speeds up by about 9.8 meters per second every second.
Anything that has mass also has gravity. Things with more mass have more gravity. Gravity gets weaker with distance, so the closer two things are, the stronger their pull. Earth's gravity pulls on the mass in your body, and that gives you weight. On a planet with less mass than Earth, you would weigh less. Your mass stays the same on Earth, on the Moon and in orbit, but your weight changes. Gravity is slightly stronger over places with more mass underground. You pull on Earth with the same force that Earth pulls on you.
Gravity does exist in space. It keeps spacecraft in orbit around Earth and Earth in orbit around the Sun. A thing that is moving keeps moving unless something pushes or pulls on it. A thing in orbit is always falling toward the planet, but it moves sideways fast enough that it never hits it. Astronauts on the space station float because they and the station are all falling together around Earth.
Gravity on the Moon is weaker than gravity on Earth. A falling apple is said to have inspired Isaac Newton to connect it with how the Moon moves. Gravity keeps the Moon moving around Earth. Isaac Newton's theory of universal gravitation was published in 1687. In 1687 Newton also explained that ocean tides come from the gravitational pull of the Sun and Moon on the oceans. The Moon's pull is the main cause of tides.
Albert Einstein described gravity as a curve in space that wraps around stars and planets. Einstein argued that the more strongly you feel gravity, the slower time passes for you. Gravity also pulls on light, so a massive thing can bend light. Massive things bend light from more distant things, an effect called gravitational lensing. A gravitational wave is an invisible ripple in space. Scientists first directly detected gravitational waves in 2015, using LIGO. The first waves LIGO found came from two black holes that collided 1.3 billion years ago.
Words to know
- mass: amount of stuff
- orbit: a path around
- tides: sea rising, falling
- lensing: light bending
