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xxvi use .net vs 2010 code 128 code set a drawer toinclude code 128 code set c on .net iOS Background: what you need to know before you start Computing The computer programs and exercises are optional, and everything you need to know about them can be downloaded from the website. You don t need to be a computer programming whiz, but you need to have had some experience with computer languages to understand the programs. However, to run the programs and see the results, play with the different possibilities anyone can do this.

. If you are co barcode 128 for .NET mfortable with these things, then let s not waste any more time thinking about how to learn about gravity. Let s do it.

. Gravity on Earth:. the inescapable force ravity is everywhere. No matter where you go, you can t seem to escape it. Pick up a stone and feel its weight.

Then carry it inside a building and feel its weight again: there won t be any difference. Take the stone into a car and speed along at 100 miles per hour on a smooth road: again there won t be any noticeable change in the stone s weight. Take the stone into the gondola of a hot-air balloon that is hovering above the Earth.

The balloon may be lighter than air, but the stone weighs just as much as before. This inescapability of gravity makes it different from all other forces of nature. Try taking a portable radio into a metal enclosure, like a car, and see what happens to its ability to pick up radio stations: it gets seriously worse.

Radio waves are one aspect of the electromagnetic force, which in other guises gives us static electricity and magnetic elds. This force does not penetrate everywhere. It can be excluded from regions if we choose the right material for the walls.

Not so for gravity. We could build a room with walls as thick as an Egyptian pyramid and made of any exotic material we choose, and yet the Earth s gravity would be right there inside, as strong as ever. Gravity acts on everything the same way.

Every body falls toward the ground, regardless of its composition. We know of no substance that accelerates upwards because of the Earth s gravity. Again this distinguishes gravity from all the other fundamental forces of Nature.

Electric charges come in two different signs, the + and - signs on a battery. A negative electron attracts a positive proton but repels other electrons. There is a simple home experiment that will show this.

If you have a clothes dryer, nd a shirt to which a couple of socks are clinging after they have been dried. Pulling the socks off separates some of the charges of the molecules of the fabric, so that the charges on the sock will attract their opposites on the shirt if they are held near enough. But the socks have the same charge and repel each other when brought together.

The existence of two signs of electric charge is responsible for the shape of our everyday world. For example, the balance between attraction and repulsion among the different charges that make up, say, a piece of wood gives it rigidity: try to stretch it and the electrons resist being pulled away from the protons; try to compress it and the electrons resist being squashed up against other electrons. Gravity allows no such ne balances, and we shall see that this means that bodies in which gravity plays a dominant role cannot be rigid.

Instead of achieving equilibrium, they have a strong tendency to collapse, sometimes even to black holes. These two facts about gravity, that it is ever-present and always attractive, might make it easy to take it for granted. It seems to be just part of the background, a constant and rather boring feature of our world.

But nothing could be further from the truth. Precisely because it penetrates everywhere and cannot be cancelled out, it. In this ch Code-128 for .NET apter: the simplest observations about gravity it is universal and attractive, and it affects all bodies in the same way have the deepest consequences. Galileo, the rst modern physicist, founded the equivalence principle on them; this will guide us throughout the book, including to black holes.

Galileo also introduced the principle of relativity, used later by Einstein. We begin here our use of computer programs for solving the equations for moving bodies. Remember, terms in boldface are in the glossary.

. The pictur e underlying the text on this page is of the famous bell tower at Pisa, where Galileo is said to have demonstrated the key to understanding gravity, that all bodies fall at the same rate. We will discuss this below. Photo by the author.

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