Saturday, June 3, 2017

Revolutions per minute

Do you know what's a unit of measurement that just hate? Revolutions per minute.

Why is it "per minute"? Why a minute? This is the only unit of measurement I'm aware of that uses one minute as its time unit. Who exactly came up with this?

The problem is that it's so nondescript. Unless you have dealt with this particular unit of measurement a lot, and have a lot of experience with it, it doesn't really tell much. It doesn't give you a mental picture of how fast something is rotating.

If something rotates, let's say, 150 revolutions per minute, how fast is that? How about 2000 RPM? Can you form a mental picture of something spinning at that speed? I can't. One minute is such a long period of time that it's really hard to just visualize it in your head.

Of course you can get revolutions per second, which is much easier to grasp as a mental picture, by dividing by 60. It's just that dividing by 60 is not an easy operation to quickly do in your head.

150 RPM is 2.5 revolutions per second. This is something much easier to mentally visualize. Just imagine in your head a disc rotating 2.5 revolutions in one second, and you'll get a pretty good concept of how fast it's spinning. Likewise 2000 RPM is about 33 revolutions per second. Again, much easier to visualize that it's pretty damn fast. The "2000" figure gave a feeling of fast spinning, but it was much more abstract and harder to concretely visualize in your head.

I just hate it when rotation speed is given in rpm. It just doesn't give me a quick mental picture of how fast it's rotating.

To be fair, there's another unit of measurement that is likewise hard to visualize without extensive amount of personal experience, namely km/h (or likewise mph if you are on the other side). Just because you can imagine something traveling eg. 10 kilometers in one hour in no way, in itself, give you a mental picture of how fast it's traveling locally. Is it walking speed? Running speed? Fast car speed? The only reason why most people are able to grasp the speed is because they have so much experience with those numbers (by driving a or traveling by car so much), and that's why they know that 10 km/h is something like jogging speed rather than something wildly different.

The problem with RPM contrasted with km/h is that the vast majority of people have almost zero personal experience with the former, while having ample experience with the latter.

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