High School Physics Lessons - Planets are Round - Practical

Why are stars and planets round?

The answer to this is related to the concepts of scale.

If you haven't already read it, please look at High School Physics Lessons - Animal Sizes - Practical to see another application of this principle.

To best explain the idea, let's consider a project. Let's say that somebody owns a bulldozer and a lot of land, and he decides he wants to pile up dirt and rocks to make the highest mountain on Earth.

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Since Mount Everest is presently the highest, and it's top is almost 6 miles above sea level, let's REALLY outdo it at make a mountain 60 miles high!

There's some Physics to consider here. For discussion sake, lets make our mountain out of the exact same materials that Mount Everest is made out of, and we'll make it so that it appears JUST LIKE Mount Everest, only bigger.

How large an area will it rest on? Well, it will be 10 times as big, north-south, and 10 times as big, east-west. This means the area of the bottom of it is 100 times the area of Mount Everest.

How much "stuff" will be in it? It will be 10 times as big, north-south, 10 times as big, east-west, and also 10 times as tall, so the "VOLUME" will be 1000 times as big as the real Mount Everest. Since we've decided to use the same kinds of rock that Mount Everest is made of, the DENSITY is the same, so the total WEIGHT of our mountain will be 1000 times that of Mount Everest.

So what does all this mean? The area of the base of our mountain will be 100 times as big, but it has 1000 times as much weight sitting on top of it.

The effect is that every square inch of the area of the base of our mountain has 10 times the weight pushing down on it than a similar square inch in the base of Mount Everest.

Why is this important?

Say you're wrestling with your brother and he pins you, with all his weight on top of you. It's uncomfortable, but you're strong, and you are not likely to be hurt. Now, say that your brother weighed 300 pounds or a thousand pounds! Or if you had ten brothers, and all of them climbed up to get stacked on top of you. You would be squished!

With that much weight on top of you, your ribs and other bones would almost certainly break under the total weight. You would wind up looking like a cartoon pancake person after they were squished!

Why is this important? It turns out that ALL materials have something called a "crushing strength". Including rib bones and granite rock and everything else. If you exceed that strength, the material gets crushed. If you exceed it even more, the material underneath kind of "flows" out. I don't even want to IMAGINE that happening to you under the stack of brothers! And, even though it doesn't seem like "solid" rock could flow, it can, just REALLY slowly!

Getting back to our giant mountain. As we keep piling more material on top, to make it taller, deep inside the rock materials will start to be crushed, so it will start slowly collapsing. As that collapsing continues, the rock at the bottom will start to very slowly flow outward. It will only move a few inches a year, so we would think that the rock is "SOLID", but it will still actually be flowing slowly!

So, even if you get enough rock piled up to make the 60 mile tall mountain, it will only be there for a few thousand years. A hundred thousand years after you built it, it would have flowed down and out to become a much WIDER mountain but much less tall.

Scientists have calculated, based on the crushing strength of granite, that the tallest mountains that could stay on Earth for more than a few hundred thousand years, would be about 7 miles tall. This is quite a bit taller than Mount Everest.

Useful Applications of Physics Knowledge

Most adults HATE the concept of Physics. If they could, they avoided it in High School and College. If they HAD to take it, Physics seems to represent a traumatic experience in their lives.

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.)

The High School Physics Lessons - Practical A of this series is:

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E-mail to: cj@mb-soft.com

C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago