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Drawing the short straw

  1. Dec 4, 2009 #1
    hey, I think its time to ask a expert on the matter now.

    I apoly for my bad English writing at first, I'm from Denmark 26 year old We all know you can take a straw and put it into a glass of water and put your finger at the hole in the other end and pull some of the water up with the straw.

    1 My question are this.. imagine this : I got ordinary straw from McDonalds I scale it up to many times its size. lets say 1 kilometer long and 50 meters in diameter. I construct a giant mechanical robotic arm out on the sea that can handle the 1 kilometer straw as a normal human would at a normal scale. Would the big difference in size matter ? or would I be able to dip it down in the sea with my robotic arm and seal the hole with my robotic finger and pull it up and take water with me up ??

    2 If the above would work and the scale don't affect the straws functions. that would make "in this case" 50 meters in diameter "lake" hover above anyone┬┤s head while swimming under the straw while its pulled up by the robotic arm. And the question : What would happen if the person swimming under the straw jumped from a boat up in the water while its hovering over him ? will the water rush out of the straw or would he be able to swim "up" ? And then..

    3 If the giant straw worked.. what would happen to the person that try to jump up from the boat into the water hovering in the straw.. I'm thinking of pressure, if the straw is 1 kilometer long and lets say its filled with water up 250 meters.. would the person be crushed by pressures when he touch the water ? because I'm thinking that he is at the bottom of the water witch the pressure much be high ?

    4 Lets say this is not possible because of the size.. if so, then if I could shrink myself so small that a normal straw would seam like a 1 kilometer long straw.. then it would work ?? If so I'm confused, because thats just a matter of size again.. just the other way around. Please help me, I'm seeing straws everywhere now ;)
  2. jcsd
  3. Dec 5, 2009 #2
    In theory, the only condition required for the water in the straw to hover is that the force pushing the water down (mg) is equal to the force pushing the water up (PA) where P is the pressure difference between the top of the water and the bottom.

    Now for a circular lake of diameter 50m, the mass per unit length will be
    Pi*(25m)^2 * 1000 kg/m^3 = 1.96*10^6 kg/m. The area of this floating lake will be Pi*(25m)^2 = 1.96*10^3. the pressure difference P required to hold up a 1 meter deep lake of this area is the gravitational force of the water divided by the area of the water: P = [1m*1.96*10^6kg/m*(9.81 m/s^2)] / 1.96*10^3 = 9810 Pa. If there is a perfect vacuum between the top of the water and the robotic arm, then the pressure difference between the top and the bottom will be 1 atm = 101300Pa, meaning a floating lake would exist with a depth of about 10m.

    I'm pretty sure my analysis is theoretically correct, but I'd wait a while for someone to call shenanigans on me before you take my word.

    2 and 3
    If a person jumped up into the water, then he/she would also have a pressure below him/her pushing them up. If the floating lake were 10m deep, they would feel the same pressure as they would feel at 10m below the surface of water at sea level.
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