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Didn’t know this was a thing. I arrived at the idea independently after noticing big airliners on landing approach look like they’re moving very slowly, but are actually moving at 150+ mph. The body-length speed explains it. We’re biomprphically wired to process speeds that way.
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Speeds expressed in body lengths per second give surprising results. A cheetah runs at 16 body lengths per second. Macropalpis mite achieves a top speed = 322 body lengths/s. The equivalent speed for a human running as fast as this mite would be 1,300 mph bit.ly/2FD02bu
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Animals we encounter ordinarily, not counting whales, are generally at most 3x-5x humans in max dimensions (elephants, giraffes) so our body-length speed sense does not feel that far off from objective speed. But huge vehicles throw us off. Oil supertankers are the most extreme.
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Even with animals, this can throw us off. Elephants seem slow and lumbering but their typical top speed of 25mph is close to human *record* of ~28mph. An average human will not outrun an average elephant. We forget their legs alone are taller than most of us.
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It's a completely useless measure though because the laws of are the same regardless of size (although some things scale with the size of the *planet* you're on) Like it takes the same amount of energy to accelerate per gram regardless of you're accelerating 1 gram or 1 ton.
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So if two things have the same *energy density* (i.e the amount of energy per cell is the same regardless of the number of cells in an animal) then they should be able to accelerate around the same rate/take about the same energy to move through the air.