One force explains why you land after a jump, why rivers run downhill, and why every planet circles the Sun. It has been pulling on you your whole life.
The answer to all three is the same word: gravity.
Gravity is the force that draws objects down toward Earth. It never switches off, it works everywhere on the planet, and it is working on you right now while you sit still.
Earth has gravity because Earth is huge and contains a huge amount of matter. All of Earth's matter gives it mass. If something has a lot of mass, it tends to have a lot of gravity and can pull things toward it.
The amount of pull between two objects is called the gravitational force. More mass, more force. Less mass, less force. If something does not have much mass, it pulls things toward it with very little force.
Distance matters too. The farther apart two objects are, the weaker the pull between them. This is why the Sun holds Neptune much more loosely than it holds Mercury.
The blue planet in the center holds a small grey moon in orbit. Slide the planet's mass up and down. Everything else stays exactly the same.
The pull gets too weak to hold on. The moon drifts away in a straight line and escapes. Not enough mass, no orbit.
The pull gets so strong the moon is dragged inward and crashes. Real orbits sit in the balance between those two extremes.
People use these words interchangeably in everyday life, but in science they mean two different things — and the difference only shows up when you leave Earth.
How much matter is in you. Measured in grams and kilograms.
Your mass is the same everywhere in the universe. On Earth, on the Moon, floating in deep space — you contain the same amount of matter.
How hard gravity pulls on that matter. Measured in pounds or newtons.
Your weight changes depending on where you are, because different places pull with different strength.
| Where you are | Your mass | What a 100 lb student weighs | Why |
|---|---|---|---|
| Earth | unchanged | 100 lb | our normal reference point |
| The Moon | unchanged | about 17 lb | far less mass than Earth |
| Mars | unchanged | about 38 lb | smaller than Earth, bigger than the Moon |
| Jupiter | unchanged | about 253 lb | enormously more mass than Earth |
For each statement, decide whether it stays the same on the Moon or changes. Tap your answer.
Round 1: one thud. The heavy book and the light eraser land together. Gravity pulls harder on the book, but the book also has more matter to get moving, and those two effects cancel out exactly.
Round 2: the crumpled ball wins easily — even though both sheets have identical mass. The difference is air resistance. The flat sheet has to push far more air out of the way on the way down.
The big idea: gravity alone would bring everything down at the same rate. Feathers fall slowly because of air, not because of gravity. On the airless Moon, an astronaut dropped a hammer and a feather together in 1971 and they landed at the same instant.
The Sun is huge and therefore has enormous gravity, which is why all of the planets in our solar system orbit the Sun. Earth is much smaller than the Sun and therefore has less gravity — which is why our small moon orbits us, but other planets do not.
Contains about 99.8% of all the mass in the solar system. Everything else — every planet, moon, asteroid and comet — shares the leftover 0.2%.
Enough mass to hold one moon and a whole atmosphere, but nowhere near enough to hold a planet.
Small enough that its gravity is about one sixth of Earth's, and too weak to hold on to an atmosphere at all.
Imagine you are standing on a very tall mountain, throwing a ball straight out. Each time, you throw it harder. Press Next to see what happens!
The string is gravity. It constantly pulls the bucket toward your hand, exactly the way the Sun pulls a planet inward.
Swing too slow and gravity wins at the top — the ball falls out. That is a moon crashing into its planet.
Let go and the bucket does not fly outward away from you — it flies off in a straight line, sideways. That straight line is the direction the planet was travelling all along. Gravity was the only thing bending it into a circle.
The Moon's gravity pulls on Earth's water. As Earth turns, coastlines pass through the bulge, giving two high tides a day. The Moon is 380,000 km away and still moves entire oceans.
Not because gravity is gone — up there it is still about 90% as strong. They float because the station and everyone inside it are falling around Earth together, endlessly missing.
Water has mass, so gravity pulls it toward Earth's center. Every river on Earth is just water taking the fastest path down.
Gas particles move fast. The Moon's weak gravity could not hold on to them, so they escaped into space long ago. Earth's stronger pull keeps our atmosphere in place.
Cover the definition, say it in your own words, then flip to compare.
Chemical Reactions. So far, every change you have seen has left the matter itself unchanged — melted ice is still water, crumpled foil is still aluminum. Next week the matter itself changes into something new.
Question for tonight: when a cake comes out of the oven, where did the flour and the raw egg go?