Take a pencil off the desk. Hold it above the desk. Drop it. The pencil clunks on the desk just like you knew it would. But why?
It falls because the pencil has mass. The Earth has a lot more mass. A force starting at the center of the huge Earth mass is attracted to the mass of that little pencil. The two objects, pencil and earth’s center, are about 4000 miles apart but still close enough to cause the earth to grab that pencil and hug it. Call it what you want — the force of gravity or gravitational attraction – but here is a good way to remember: Matter likes matter. Every piece of matter in the Universe likes every other piece of matter in the Universe. Maybe it should be called the “Hey, let’s get together!” syndrome.
Newton called “liking each other” gravitational attraction, a much more boring name. His equation said the bigger the two masses were and the closer together they got, the more would be the force to pull them together.
Gravitational attraction at work: For about a half billion years after Big Bang, the sky was filled with a sort of dense fog but was also very, very bright. Then all went dark; light returning later from an exploding star.
Around seven billion years later, a giant cloud of matter formed the Milky Way. Our sun was there, one of the new folks on the block; its tremendous gravity had convinced a whole lot of other matter to tag along.. Out of that came our planet, third from the Sun with the moon (about the size of the planet Mercury) flying around it.
Picture it. Our sun was in the Milky Way but all matter was still trying to zoom away from the Big Bang explosion site. Our planet has been attracted to this huge chunk of matter (our sun.) Gravitational attraction existed; yet everything – Sun and planet surrounding it – still had the momentum to fly in a straight line away from the Big Bang explosion site.
But the Sun has a mass much, much larger than ours. The force of attraction between our earth and that much-bigger sun was strong. A real conundrum! Momentum wanted the earth to keep going in a straight line. But gravitational attraction wanted to bring earth down and to crash into the Sun. Gravity and momentum are in a fight to the death!
Eureka! A tie! Earth’s inertia to fly off further into space was balanced by the gravitational attraction between Earth’s mass and the Sun’s mass. And, since Newton’s rule that some external force is needed change things, the tie will continue. The same type of combination of forces keeps the Moon in a pretty constant orbit around us.
The Earth’s hot center is metallic, the source of Earth’s magnetic field. The magnetic field is what pulls your compass needle to N (north), but more importantly, that magnetic field shields Earth from those death rays sent down by the Sun. A strong burst from the Sun, though, can send enough through to provide us with those magnificent Northern Lights.
Our planet’s surface was dry at first but water was in the atmosphere. As our earth cooled, the water vapor in the atmosphere became water; then came the oceans. Many scientists feel a good deal of our water came from colliding comets made of frozen water. Wherever the water came from, for a long time the Earth was just one long, uninterrupted ocean of water.
Volcanoes interrupted. Sometimes that collection of lava got high enough to peak over the top of the constant ocean. Volcanoes, however, were not the only source of land. What else could there be? Colliding candy bars.
Imagine taking a thin chocolate candy bar and, holding both ends. Apply a little pressure. The bar splits into a bunch of funny-shaped flat surfaces. Since our Earth is a sphere, how could those funny-shaped flat plates cover the earth? The only way it worked was to have gaps between them.
As the earth’s molten surface cooled, the hard crust—now eight of them. Think of a whole bunch of broken chocolate pieces pressed all around a round scoop of ice cream. Those eight plates covering the earth float on top of the boiling molten center of the earth, protecting what is above from that intense heat.
“Float.” Those plates slowly floated around, large and small, banging into one another, parting company, crashed again (sometimes hard enough to lift up today’s mountains), and kept moving – as they still do today! (Plate-drivers must have been texting while they drove.) The very ground on which you live has been all over the globe – below the equator at times, close to the South Pole at times and really, really cold, near the equator and really hot, and finally setting here at the mid-latitudes of the Northern Hemisphere. The continents began to take their places.
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