Learning to be Terse

This man can walk on water!

Posted in Science by Croor Singh on October 8, 2010

Okay, I’m kidding. It’s not the man that’s the cause, it’s the water. And it’s not really water. It’s a mix of cornstarch and water. Here’s the video.

The fluid is supporting the guy’s weight. But how is that possible? The Stokes equation for the shear-stress in a Newtonian fluid is this:

Which means, basically, that the shear-rate is proportional to the applied stress. The constant of proportionality, the viscosity, can be measured to be quite small. You wouldn’t be able to stand on water, for example; the viscosity isn’t nearly enough to support your weight. So what gives (or holds!)? How is the fellow walking on water?

The answer is to note that the guy isn’t ‘walking’ on water: he’s running. And when he walks slowly or stands still, he sinks. The reason our model can’t predict this is because, as is usually the case, the model we’re working with is incomplete. The fluid is non-Newtonian. In this particular instance, it’s called a dilatant or a shear-thickening fluid. A better model for this fluid is this:

The first term on the LHS is the same as that for a Newtonian fluid. The second term is what you would expect with a regular solid. Because this mixture has properties of both liquids and solids, it’s called a visco-elastic material. The model itself was proposed by Maxwell, and is named after him.

Now we see what happens. Because the rate of increase in the stress is much greater than the stress itself (this is when the guy is running), the fluid behaves like a solid, and can therefore support his weight. If, on the other hand, the guy stands still, the rate of increase of the stress is nil (or small), and the mixture starts to behave like a (fairly viscous, as you can see) liquid.

Good. Now that you know of the Maxwell model, you can explain why it is that people can break bones if they land the wrong way on water. To wit: divers are taught to go in arms (or legs) first, so that the rate of increase of stress isn’t too high when the torso gets to the water’s surface. Alternatively, you could fall flat on your back onto the water… you know, if you want a raw back that hurts like it’s been whipped!

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18 Responses

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  1. Consti said, on October 8, 2010 at 3:30 pm

    Nerd Jesus walked on a Non-Newtonian fluid for your sins.

  2. Croor Singh said, on October 8, 2010 at 3:35 pm

    Indeed! B=D

  3. strawberrymargaritas said, on October 8, 2010 at 5:43 pm

    lol lol.

  4. Bhagwad Jal Park said, on October 8, 2010 at 6:25 pm

    Just added your blogs to Networked Blogs on Facebook. You can claim yourself as the author here: http://apps.facebook.com/blognetworks/blog/learning_to_be_terse/

  5. Croor Singh said, on October 8, 2010 at 7:10 pm

    Yes. This is now a networked blog.

    You can now follow this blog on Facebook. Go to my profile page, and find the tab named ‘Blog’.

    I’ll put up a link to do all this here as well, sometime.

  6. Adarsh VE said, on October 8, 2010 at 9:01 pm

    You should see this :

  7. Croor Singh said, on October 8, 2010 at 10:23 pm

    Holy mother of god!

    No, seriously. Does the holy mother of god know about this? Isn’t there some copyright on this kind of thing or something?

  8. Nair said, on October 8, 2010 at 10:47 pm

    You should try this stunt in the dead sea!

  9. Croor Singh said, on October 8, 2010 at 11:38 pm

    Aarrr! How dare you call the true act of the one true loving goD to save all humanity from… something… a stunt? Aarrr!

  10. Croor Singh said, on October 10, 2010 at 12:08 am

    Jesus didn’t walk on water? Moses didn’t part the red sea? Santa isn’t real?

    You cruel cruel woman! How could you?!

  11. Sailaja said, on October 10, 2010 at 12:53 pm

    Haha :D Of course moses didn’t part the red sea ;)

  12. Praneeth said, on October 10, 2010 at 8:53 pm

    @ Croor.. Its GOD who parted the RED SEA.. wen Moses prayed while delivering Isrealites out of Egypt

  13. Croor Singh said, on October 10, 2010 at 9:01 pm

    To repeat myself, Sailaja, you cruel cruel woman! How could you do that to Praneeth?

    @Praneeth, seriously, stop dude. It’s not even funny anymore. goD farted and the whole world held their noses. When the stench had subsided, they all heaved such a sigh of relief that the sea split in two. There. That’s as believable as what’s there in the bible.

  14. Praneeth said, on October 10, 2010 at 9:45 pm

    Ok da.. delete delete all those comments that you can.. & you want to..

    “He who has ears, let him hear..” & remember wen u r trying to argue, take all facts into consideration…. wen ur blogging with comment section open.. ur just making a debate.. dont jus start criticizing others jus because u can… & also remember If ur sooo scientific, u gotto weigh all the facts before making a comment..

    GOOD BYE

  15. Croor Singh said, on October 10, 2010 at 10:38 pm

    I AM asking people to comment on the blog. I am, however, also asking that the comments at least be based in reality.

    Thinking that the sea split in two to let some people go through, because some book says so is NOT weighing the facts. It is, on the contrary, the definition of not weighing fact, and replacing it with some mix of fantasy and delusion.

  16. Dhananjay said, on October 11, 2010 at 2:08 pm

    “based on reality” instead of “based in reality”. And I see, you people are playing ‘The Man From Earth’ here in your blog site eh. :)

  17. Croor Singh said, on October 11, 2010 at 5:17 pm

    We passed the ‘Man from Earth’ stage a long time ago!


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