Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

In a video (Here), I saw crocodiles jump vertically about three meters without using any solid surface. The wonderful thing is that when they start to jump, their vertical velocity is approximately zero, unlike fish who jump using initial velocity. It seems that crocodiles create an upward force that counteracts gravity, because when they are rising, their velocity seems to be constant.

How is this possible? Could anyone explain this phenomenon using physics laws?

share|improve this question
6  
Action and reaction: the stronger they push the water down, the stronger the upward thrust. Although they seem very heavy and immobile at first, their weight and size allows them to displace huge amounts of water. –  Phonon Aug 7 at 11:33
2  
@Phonon That should be an answer, not a comment. –  rob Aug 7 at 12:50
    
@rob indeed, since there were already a couple of good answers, I just wanted to remind him of "action-reaction", which may not have been explicitly mentioned in the answers. –  Phonon Aug 7 at 13:02
    
@Phonon therefore, should of been provided as an answer –  Adsy Aug 8 at 10:24

3 Answers 3

up vote 21 down vote accepted

If you look closely at the crocodiles' tails you'll see that they wave their tails from side to side to provide propulsion for the jump. Compare this to a fish swimming:

Fish swimming

The side to side motion of the fish's tail propels it forward, and the crocodiles are using exactly the same sort of side to side motion to propel themselves upwards.

share|improve this answer

I've recently started trying to swim the butterfly. Unlike other swimming strokes, there doesn't seem to be any way to "go easy" and do a relaxing length of the pool: if I want to get my face out of the water to breathe, I essentially have to use the water to do a push-up.

Now if a scrawny guy like me can lift his chin a few inches out of the water using his scrawny arms, it stands to reason that a crocodile with a meter of muscle in his tail should be able to do better.

If you look closely at the videos you'll notice that the "constant velocity" part of the ascent only lasts while the thick part of the tail is underwater. Once the crocodile is far enough up that only the skinny end of its tail remains in the water, the ascent slows.

share|improve this answer
    
+1 for humor, hah! –  Danu Aug 7 at 13:01
1  
+1 for the best stroke. Also a possibly more direct analogy is people doing vertical dolphin kick drills. –  Chris White Aug 7 at 20:55

The upwards force comes from the rather violent tail movement. When the rest of the body is out of the water, the tail still acts sort of like a hydrofoil pushing the crocodile upwards, only not with a linear but oscillating motion, and obviously it's rather instable but enough to get the whole animal up in the air for a short while.

share|improve this answer
2  
I don't think that hydrofoil is the correct analogy here. –  Dave Aug 7 at 13:19
    
I don't think that hydrofoil is the correct analogy either, in the sense that it's not an analogy, it's an accurate description of the process, using the general fluid mechanics sense of the term. –  Sean D Aug 7 at 23:33
1  
Agreed, "hydrofoil" is not an analogy here. It's exactly what's happening. –  slebetman Aug 8 at 2:11
    
Are there really subfields of fluid mechanics that define a foil as something that provides thrust? Because otherwise the tail's classification as a hydrofoil (i.e. "a thing in the water") is really irrelevant. –  Chris White Aug 8 at 17:37
    
@ChrisWhite: if you consider the resultant force not so much as thrust, rather as lift, then it's perfectly obvious to call it hydrofoil. As to why I considerer it as lift here, the question was about "counteracting gravity" – though that in itself is somewhat unfortunately put of course, since in fact there is quite some acceleration going on. –  leftaroundabout Aug 9 at 0:46

protected by Qmechanic Aug 8 at 12:54

Thank you for your interest in this question. Because it has attracted low-quality answers, posting an answer now requires 10 reputation on this site.

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.