29 dicembre 2016 alle 0:45 #136
Gravity is a force pulling together all matter (which is anything you can physically touch). The more matter, the more gravity, so things that have a lot of matter such as planets and moons and stars pull more strongly.
Mass is how we measure the amount of matter in something. The more massive something is, the more of a gravitational pull it exerts. As we walk on the surface of the Earth, it pulls on us, and we pull back. But since the Earth is so much more massive than we are, the pull from us is not strong enough to move the Earth, while the pull from the Earth can make us fall flat on our faces.
In addition to depending on the amount of mass, gravity also depends on how far you are from something. This is why we are stuck to the surface of the Earth instead of being pulled off into the Sun, which has many more times the gravity of the Earth.
Gravity has played a big part in making the universe the way it is. Gravity is what makes pieces of matter clump together into planets, moons, and stars. Gravity is what makes the planets orbit the stars–like Earth orbits our star, the Sun. Gravity is what makes the stars clump together in huge, swirling galaxies.
You see gravity at work any time you drop a book, step on a scale or toss a ball up into the air. It’s such a constant presence in our lives, we seldom marvel at the mystery of it — but even with several well-received theories out there attempting to explain why a book falls to the ground (and at the same rate as a pebble or a couch, at that), they’re still just theories. The mystery of gravity’s pull is pretty much intact.
A great scientist, Albert Einstein, who lived in the 20th century, had a new idea about gravity. He thought that gravity is what happens when space itself is curved or warped around a mass, such as a star or a planet. Thus, a star or planet would cause kind of a dip in space so that any other object that came too near would tend to fall into the dip.
Quite a number of experiments show that Einstein was right about this idea and a lot of others. But there are questions for which even Einstein had no answers.
For example, if gravity is a force that causes all matter to be attracted to all other matter, why are atoms mostly empty space inside? (There is really hardly any actual matter in an atom!) How are the forces that hold atoms together different from gravity? Is it possible that all the forces we see at work in nature are really different sides of the same basic force or structure?
These are big questions that scientists and ordinary people like us have wondered about for a long time. For a long time we haven’t known how to go about finding the answers, other than trying to work things out on paper.
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