Calculating Speed of Antelope in an image

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29 points·ruchi··6 comments

A nice back-of-envelope calculation. Someone posted a picture showing an antelope running through shallow water. Someone else realised that you can use just that single picture to estimate how fast the antelope is moving. The clever bit is estimating the speed of the water waves you can see spreading out from places where the antelope touched down.

However, I am unconvinced by some details. For instance, I think s/he overestimated the amount of water wave movement by maybe a factor of 2, the wavelength looks way too large too, and I suspect the water is deeper than s/he assumed. My estimate of the antelope's speed is more like 40mph, which is still very impressive given that it's running through water.

Why do you suppose that the water is deeper than 1 ft?

I said "suspect" rather than "suppose", which may be a useful clue: It just happens to look that way to me. I have no particular reason to trust my judgement on this. I'm sure it isn't much deeper than 1ft, in any case. (Nor much shallower, for that matter.)

If it were much deeper, I'm not sure the animal could have sustained it's velocity. I cannot walk normally in six inches of ocean water.

Yes, but you're (presumably) h. sapiens. You've got big flat feet that displace a lot of water (twice) every time you take a step.

An antelope's hooves are hard and narrow, so they generate less turbulence going into the water. I'm not sure how the antelope deals with the mud on the bottom of the river, though.

Deer hooves open forming a V shape when touching the floor, that's how they don't get stuck when drinking water from a lake. I would guess speed is also important.

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