accelerationHow quickly a moving thing's velocity is changing. Near Earth's surface, with air ignored, each second of falling adds about 9.8 meters per second to an object's speed.
air resistanceThe slowing force that air puts on a moving body. On the moon, where there is no air to resist, an astronaut dropped a hammer and a feather in 1971 and they fell at the same rate.
attractionA pulling together, as by gravity or by a magnet. A person pulls on Earth with the same force that Earth pulls on the person. Earth has so much more mass that the pull has no real effect on it.
forceA cause of change in a body's motion or its shape. Force is measured in newtons. One newton speeds up a mass of one kilogram by one meter per second every second.
free fallMoving under gravity's pull and no other force. By this meaning a ball thrown upward is in free fall even on the way up, and so is the moon as it circles Earth.
Show all 36 terms (31 more)
Gravity basics, continued
gravityThe pull that every mass has on every other mass. Its name comes from the Latin gravitas, weight. It grows weaker with distance, but its reach has no end.
inertiaThe way a body keeps on resting or keeps on moving. It is the subject of Newton's first law of motion. An orbit is a tug-of-war between a satellite's straight-line motion and the pull of gravity.
kilogramThe metric measure, written kg, used for mass. A kilogram is 1,000 grams. When the unit was created in 1795 it was tied to the mass of a liter of water.
massThe measure of matter in a body, unchanged by place. Mass is measured in kilograms. It is also a measure of how hard an object is to speed up.
orbitThe looping path of a body held by another's gravity. A satellite in orbit never stops falling toward its planet, but it moves sideways fast enough that it keeps missing.
tideThe regular lift and drop of sea level along a coast. The moon's gravity raises a bulge of ocean on the near side of Earth and another on the far side.
weightThe downward force that gravity puts on a body. A mass of one kilogram weighs about 9.8 newtons on Earth. On Mars, with its weaker gravity, the same mass weighs less.
Falling, weighing and energy 12
altitudeDistance straight up from the ground or the sea. Gravity weakens with altitude, though slowly: someone who weighs 100 pounds at ground level would still weigh 90 at the height of the space station.
center of gravityThe single point where a body's weight seems to act. It marks the average position of an object's weight. A thrown object turns about this point as it flies.
g-forceAcceleration felt as weight, counted in units of g. Anything at rest on the ground is under 1 g. Pilots in aerobatic flight and the riders of a rocket at launch feel more.
gravimeterA device that measures the local pull of gravity. In one common kind a weight hangs on a spring, and the spring's change in length shows how strongly gravity pulls.
gravitational constantThe number, written G, that sets gravity's strength. Newton's law of gravity needs it to turn masses and a distance into a force. The first measurement good to about 1 percent came from an experiment by Henry Cavendish in 1798.
jouleAbout the energy needed to lift an apple one meter. It is the unit for both energy and work. One newton of force pushing something one meter does one joule of work.
kinetic energyThe energy a body carries by virtue of its speed. The name is from the Greek kinesis, motion. A falling object gains kinetic energy as it loses height.
pendulumA bob that swings under gravity from a fixed point. The time of one full swing depends mostly on its length. Pendulum clocks, invented in 1656, kept the world on time for centuries.
potential energyEnergy held in store by a thing's height or position. Lifting a book onto a high shelf gives it gravitational potential energy. If it falls, that energy turns into energy of motion.
projectileA ball, arrow or stone in flight once it is let go. A kicked football and a thrown baseball are projectiles. With air left out, the path of a projectile is a parabola.
terminal velocityThe top speed a falling object reaches in air. It comes when the drag of the air has grown to match the weight of the object. From then on the object falls at a steady speed.
weightlessnessThe feeling of having no weight, as in free fall. Astronauts float because they and their craft are falling around Earth together.
Gravity in space and at sea 12
binary starA pair of suns circling one shared balance point. Watching the pair lets astronomers measure the stars' masses directly.
black holeA pit of gravity so deep that light cannot climb out. The idea that gravity could be strong enough to hold back light was first put forward in 1784.
cometAn icy visitor whose stretched orbit nears the sun. Most comets follow long, stretched orbits that swing in close to the sun and back out to the far reaches of the solar system.
dwarf planetA world like Pluto: round, but with a crowded orbit. Its gravity is strong enough to pull it into a ball but not to clear other objects from its path around the sun. The class dates from 2006.
event horizonA black hole's edge, the boundary of no return. It is not a solid surface. The term was coined in the 1950s.
galaxyAn island of stars, gas and dust adrift in space. Most large galaxies have a supermassive black hole at the center. Our solar system lies in the galaxy called the Milky Way.
gravitational lensA mass that bends the light of things far behind it. A cluster of galaxies can act as one, magnifying a more distant galaxy and stretching its image into an arc or a ring.
high tideThe highest level the sea reaches as a tide comes in. On most coasts the sea reaches high tide twice a day, 12 hours and 25 minutes apart.
low tideThe lowest level the sea falls to as a tide goes out. The tide day runs 24 hours and 50 minutes, because the moon moves along its orbit while Earth turns.
nebulaA vast cloud of gas in space, often a star nursery. Clumps in the cloud draw in more matter until they are dense enough to become stars. In Latin, nebula means cloud or fog.
neutron starThe tiny, dense core left by an exploded giant star. It packs about 1.4 times the mass of the sun into a ball about 20 kilometers across. Only a black hole is denser.
starA shining sphere like the sun, bound by its gravity. A star's life begins when gravity makes a cloud of gas collapse. The sun is the star nearest Earth.
How this list was compiled: Term list chosen for this site on 2026-10-12. Each meaning is written in our own words and was checked that day against the article on the term in the English Wikipedia (https://en.wikipedia.org/), whose text is under the Creative Commons Attribution-ShareAlike license: Gravity, Newton's law of universal gravitation, Newton (unit), Mass, Weight, Mass versus weight, Free fall, Acceleration, Gravity of Earth, Drag (physics), Apollo 15, Orbit, Tide, Kilogram, Inertia, Weightlessness, Center of mass, G-force, Gravimetry, Projectile, Trajectory, Pendulum, Potential energy, Gravitational energy, Kinetic energy, Joule, Altitude, Gravitational constant, Star, Nebula, Comet, Dwarf planet, Binary star, Neutron star, Event horizon, Gravitational lens and Gravitation of the Moon. Force, terminal velocity, gravimeter, weightlessness, high tide and low tide were read in Wiktionary, the free dictionary (https://en.wiktionary.org/). Gravity, orbits, weightlessness, black holes, galaxies, nebulae, comets, dwarf planets, stars, gravitational lenses and the tides were checked a second time against pages of the National Aeronautics and Space Administration: NASA Space Place (https://spaceplace.nasa.gov/what-is-gravity/en/, What Is Gravity?, and its pages on orbits, black holes, galaxies, nebulae and comets), What Is Microgravity? and What Is an Orbit? (https://www.nasa.gov/learning-resources/for-kids-and-students/), NASA Science (https://science.nasa.gov/universe/black-holes/, and its pages on galaxies, stars, dwarf planets, comets, tides and Hubble's gravitational lenses) and the Beginner's Guide to Aeronautics of the Glenn Research Center (https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/termvel/, Terminal Velocity, and Center of Gravity). High and low tides were checked against the tides tutorial of the National Ocean Service of the National Oceanic and Atmospheric Administration (https://oceanservice.noaa.gov/education/tutorial_tides/tides05_lunarday.html, Frequency of Tides: The Lunar Day). The rows for force, terminal velocity, black hole and galaxy rest on Wiktionary and on the NASA pages, not on the Wikipedia articles of those names. The fact after each meaning is from the same pages. Licence: own work
How it works
This gravity vocabulary worksheet prints the first twelve words of a gravity unit, among them gravity, mass, weight, attraction, free fall, acceleration, orbit and tide, with their meanings mixed up on the right. A key is the last page.
Read the full guide
The rest of the gravity unit: falling, weighing and energy
Set opens on Gravity basics. The second set has terminal velocity, weightlessness, center of gravity, g-force, gravimeter, projectile, pendulum, potential energy, kinetic energy, joule, altitude and gravitational constant.
Gravity in space and at sea
The third set has star, galaxy, nebula, comet, dwarf planet, binary star, neutron star, black hole, event horizon, gravitational lens, high tide and low tide.
Friction, thrust, lift and the simple machines are not terms here: they are on the force and motion word search.
Questions and answers
Is gravity a force?
In Newton's physics, yes: a pull between any two masses. In Einstein's general relativity of 1915, gravity is the curving of space and time around a mass. For most purposes Newton's law still describes gravity well enough.
Who discovered gravity?
Nobody discovered it, since things have always fallen. Isaac Newton gave the first law for it in his Principia of 1687: every mass attracts every other, and the pull weakens with distance. He used it to explain the tides.
Why is gravity weaker on the moon?
The moon has much less mass than Earth, and the pull of a body depends on its mass. On the moon's surface things weigh about one sixth of what they weigh on Earth.