Why is the Earth round? Sounds like a simple question! The answer
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In science fiction, there are sometimes planets that are shown as having points or corners. Is that possible?
We need to first think about some other things first. Think about a cube that is one foot along each edge. We call its volume one cubic foot and the area of any of its six surfaces one square foot.
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The area of each surface is two feet by two feet now, or FOUR square feet. The amount of space inside it (the volume) is two feet by two feet by two feet, or EIGHT cubic feet. If you don't follow that, think about how many of the smaller cubes would be necessary to make one of the bigger cube. A layer of them would have 4 smaller cubes and there would have to be two layers, or eight all together!
What in the world does this have to do with planets with corners? (I'm getting to that!)
Even though we were just talking about cubes, somewhere in Geometry there's a proof that any shaped object could be duplicated by a whole bunch of tiny cubes. So, ANY shaped object would follow this rule, including rocks and mountains!
Nearly all the rocks that have been found on the Earth are made up of pretty much the same basic materials. They all have approximately the same actual strength. All materials have a strength limit, called the CRUSHING STRENGTH. This includes every kind of rock. If you exceed the crushing strength, the structure of the rock disintegrates and the particles of it begin to slowly flow around, like they were a liquid! Well, like a really THICK liquid, like molasses. Like molasses in winter when it hardly flows at all. Under those conditions, you could turn a bowl of molasses sideways and then not see it spill out. But, a few hours later, you would find that it had moved downward.
Rock SEEMS really solid to us. It's hard to imagine it ever "flowing." But, over thousands of years, it can slowly flow.
Getting back to the cubes and stuff. Imagine a cube TEN times as big. It would have a VOLUME of 10 times 10 times 10 or 1,000 times as much. Since it was made out of the same (rock) material, it would WEIGH 1,000 times as much. Now, how much area is there underneath the giant cube? Only 10 times 10 times the area of the original one. So we would have 1,000 times as much weight pressing down on an area 100 times as big. Do you see that each particle of rock underneath it is subjected to TEN times as much weight?
Granite is a REALLY sturdy kind of rock. But the logic presented above applies to granite. So, if we started piling up blocks of granite, we would eventually cause so much pressure (weight) to exist on the granite at the very bottom of the pile, where the crushing strength would be exceeded, and the particles of that granite would begin to flow (very slowly) outward!
Since we know how strong granite actually is, and we know how strong the Earth's gravity is, we can calculate how tall a pile of granite could be. It turns out to be less than 10 miles tall. The tallest mountain on Earth is Mount Everest, which is about 6 miles high. Since the Earth is about 8,000 miles in diameter, a mountain 6 or 10 miles high wouldn't noticeably change its round (spherical) shape.
All of these comments apply to ALL very large objects in the Universe. In other words, any object as large or larger than the Earth, cannot have a (stable) mountain or irregularity much taller than our Mt. Everest. Therefore, ALL of the really large objects in the Universe HAVE to be basically round, with NO points.
By the way, if the object is smaller, like a moon or asteroid, the gravity of that object is much less than on Earth. Even though the materials are similar to the granite, the much lower gravity causes the WEIGHT of the mountain to be much less. Therefore, an asteroid 40 miles in diameter COULD have a mountain that was 40 miles tall. So, smaller asteroids are not necessarily round. Many of the smaller ones have been found to be unusually shaped.
This is the case primarily because no one (and no textbook) showed them the incredible usefulness a moderate knowledge of Physics can be.
This series of lessons is meant to correct that situation. Students in High School or College Physics should be able to benefit from and EVEN ENJOY (!!) these Physics lessons. The lessons should help clarify the usages of a lot of those dry subjects and equations the teacher or professor tries to ram down your throat. These lessons are freely made available to teachers and professors for use as they desire, either on the InterNet or in the classroom.
(The preceding paragraphs appears in each lesson, in the event that someone happens to find a single lesson from this series as a result of a search-engine search.)
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This subject presentation was last updated on - - The High School Physics Lessons - Practical A
C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago