Gravity

From ForcaWiki, the simple encyclopedia

Throw a ball straight up as hard as you can. It always comes back down. Something invisible is pulling it back toward the ground the whole time, like a hand you cannot see that never gets tired. That pull is called gravity.

Everything made of material has a little bit of this pull, and the pull gets stronger the more "stuff" is packed into an object. Scientists call the amount of stuff its "mass." Here is the surprising part: you pull on the Earth exactly as hard as the Earth pulls on you. But the Earth is so enormously heavy that your pull barely nudges it, while its pull yanks you straight down. That is why you stay planted on the ground instead of drifting off like a balloon.

Which things pull the hardest?

The more mass something has, the harder it pulls. A huge planet pulls much harder than a small moon. Our Moon has far less mass than Earth, so its gravity is weaker, only about one sixth as strong as ours. That is why astronauts on the Moon could bounce around in giant leaps even while wearing heavy suits.

The Sun has more mass than anything else near us, so it has the strongest pull in our whole neighborhood. Its gravity holds the Earth in a giant loop around it, the same way a string keeps a ball circling your hand when you swing it. The Moon's gravity reaches all the way to Earth's oceans and tugs on the water, and that is what causes the tides at the beach. The strongest gravity in the entire universe belongs to Black Holes, which pull so hard that not even light can get away.

What if gravity switched off?

Life would get very silly very fast. Your juice would drift out of its cup in wobbling blobs. Your dog would float up off the sidewalk. Nothing would stay on the ground at all.

Here is a surprise, though: astronauts on the space station float around, but not because gravity is switched off up there. At that height, Earth's gravity is still almost as strong as it is in your backyard. They float because the station is constantly falling around the Earth, like a roller coaster drop that never ends, and the astronauts and their juice are falling right alongside it. When everything falls together, it all appears to float. That is why they sip from sealed pouches instead of open cups.

Fun facts

• A scientist named Isaac Newton worked out the first great explanation of gravity in the 1600s, and the story goes that he started wondering about it after watching an apple fall from a tree.
• On the Moon there is no air to slow a feather down, so a dropped feather and a dropped hammer hit the ground at exactly the same moment. An astronaut really tried this in 1971, and it worked.
• Gravity is also why rivers flow downhill and why rain falls, because everything loose is always being pulled toward the center of the Earth.