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There are various reasons why water isn't a good basis for length measurements. In the long term, the most important is that water evaporates. To appreciate this, keep in mind that you can't just weigh out a kilogram of water and then somehow make a perfect cube container, then measure the sides. At the very least, dealing with the meniscus at the top would ...

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When we pick standards for units it's important to pick standards that are as percisely defined as possible and measureable to very high precision in the lab. So for example the second is defined as 9192631770 times the period of the radiation emitted from a specific transition of the caesium atom. This is easily measured and indeed thousands of atomic ...

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Physics processes are independent on the measurement units that you use. Your question is not about physics but about math. If you square a number less than one it will result in a smaller number, if you square a number larger than one you will get a larger number. But changing units will not change anything. For instance, suppose you have a square that is ...

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I'm not sure about your rain comment and how you were taught rain formed, but I can give you a bit of info on units. It doesn't matter what length you choose to be the base unit, as long as you are consistent. Right now our definition of the meter is the length light travels in $1/c$ seconds, where $c$ is the defined speed of light. You have to ...

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As you correctly note, the definitions of the kilogram, meter and second are necessary to define the ampere. However, it doesn't mean that they are sufficient for that purpose. Formally, what you require is that the seven base dimensions of the SI be linearly independent in the vector space I describe in this answer. Each SI unit introduces something new to ...

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So a unit provides two different things: type and scale. When you say e.g. $1000 \text{ ms} = 1\text{ s}$ you are communicating both a type-equality and a scale factor between them. Type, in physics, is really subtle. Yes, we can make do with a lot fewer types than the SI system uses; in fact physicists using Planck units frequently make do with only one ...

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Only the definitions of one second, one Kelvin and one kilogram are completely independent of each other (see this scheme). All other base units are derived from these three units. In earlier days, the meter was just an etalon which was defined as 1 meter. Currently it is defined via the second with the help of the speed of light. It's derived from the ...

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