What Is the Kilogram?
Alright, let’s be real. There’s a lot of weird shenanigans when it comes to units of measurement. We’re talking kernels, stones, fathoms, and all sorts of weird unit nonsense. We’ve complained about the US Customary System for measurements in various posts on the blog, but who can blame us? It’s all based on some dead dude’s foot. Anyway, what about the metric system? People talk about this thing named “The Kilogram,” (not the unit, but an actual object). So what exactly is the kilogram?
Further Reading: Why Do we Have both Metric and Imperial Systems?
Kilogram: The Unit
Just to benchmark, we should make sure we’re all on the same page with what a kilogram is. It’s roughly 1000 grams (how convenient it’s all in multiples of 10). If you like pounds, 1 kilo is about 2.2 pounds. Grams are the base unit for kilograms, so it’s kind of important to establish that baseline.
Oh yeah, the kilogram is also a measurement of mass. So when you multiply that value by the acceleration of gravity on Earth, you get weight. In metric though, we normally describe weight as mass.
There isn’t really that much more to the kilogram than that. It’s a way to describe how much mass something has. Which is another way to describe how much something weighs. The difference between the terms “mass” and “weight” isn’t significant for your day to day life–since it’s relative to the acceleration of gravity. Since the Earth’s acceleration of gravity is basically a constant, it’s not of concern to us.
Where the distinction does come to concern is in stuff like space, where there is no acceleration of gravity. You can read more about that here on the blog. It’s the same reason for why you technically weigh less on Mars, despite being of the same mass.
Kilogram: The Object
Okay, but what we’re really interested in is all the weird technicalities of the kilogram.
So, like we did before, let’s start with the gram. What defines a gram? It sounds weird and funky, but the science is sound we promise.
A gram was defined as the weight of a cubic centimeter of water. That’s pure water, so not the mineral infused stuff that comes out of the tap. We redefined it though, since the kilogram became more widely used. So now, a gram is actually 1/1000th of a kilogram, and not the other way around. You can thank the International Bureau of Weights and Measures.
The kilogram is now defined in terms of the speed of light, which is also where the meter and second originate from. It’s made into the kilogram with the Planck constant. To a non-physicist, it relates a photon’s energy to a frequency in Joule-seconds (Joules times seconds).
All this happened on the 20th of May, 2019, mostly because of the object we were talking about earlier. Basically, we had this block made of platinum and iridium (but 90% platinum) called the International Prototype of the Kilogram. The original is in France, and there are a handful of copies floating around. We started using it in 1889. But then we realized that objects decay over time, hence how many times we’ve tried to redefine the kilogram, having now just settled on the one we have now.
At least it’s a lot better than “we saw a foot once,” we suppose.
Related post: The Redefined International System of Units