By Aneek

2015-07-11 04:50:09 8 Comments

The term escape velocity is quite common to us. But we know velocity is a dimension dependent on the direction. But the escape velocity has same value irrespective of from where it is thrown from the earth. So its just the magnitude that matters, not the direction. In that sense it should be given the name as escape speed. But it is not given so, can anyone explain?


@John Duffield 2015-07-11 12:22:36

As others have said, escape velocity is a speed, not a velocity. As to why, see the etymology of the word velocity:

early 15c., from Latin velocitatem (nominative velocitas) "swiftness, speed," from velox (genitive velocis) "swift, speedy, rapid, quick," of uncertain origin.

Velocity used to mean speed, and we still say things like "high velocity bullet". The vector-quantity meaning came later. Interestingly, when you look at translations of Einstein's papers such as here, you see the word velocity used instead of the word speed. Then when you backtrack to the original German, you find the original word was Geschwindigkeit. Native German speakers have advised me that it means both speed and velocity.

@John Rennie 2015-07-11 05:12:22

Yes, escape velocity should really be escape speed. The Wikipedia article on escape velocity states this explicitly.

I doubt there is any logical reason for using the term escape velocity and I suspect it is an accident of history. You might want to ask on the History of Science SE how the term originated - a quick Google failed to retrieve any information about the origin of the phrase.

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