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Forces while squeezing toothpase tube

  1. Sep 15, 2014 #1
    When I squeeze tube of toothpaste, I am working with 2 squeeze forces toward tube. Why it moves vertically from horizontally applied force (when I think of paste as a group of particles inside a tube I cannot imagine that) ?

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  2. jcsd
  3. Sep 16, 2014 #2

    davenn

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    hi jyhghy

    welcome to PF

    vertical , horizontal its irrelevant. That's just the way you are holding the tube
    if the tube was lying flat on the table and you squeezed the sides, do you think it would make any difference?

    if the cap was on the tube and you were squeezing the tube, the pressure is built up without any opening to release it. take the cap off and the pressure can be released. That is regardless of the orientation of the tube (up, down or sideways)

    what do you think ?

    cheers
    Dae
     
  4. Sep 16, 2014 #3

    Nugatory

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    I think that you're asking how a force applied in the direction of the red arrow can cause toothpaste to move in the direction of the blue arrow? Squeezing the tube increases the pressure in the tube, which pushes equally in all directions. The blob of toothpaste in the neck of the tube is subject to this pressure from one side but the other so moves down the neck and out of the tube.
     
  5. Sep 16, 2014 #4

    A.T.

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    The red arrows are just the forces you apply to the tube. The forces that the tube applies to the paste are distributed all over its inner surface. But there are none at the opening, so that's where the paste goes.

    If you have problems visualizing how fluids distribute directed forces into pressure that acts in all directions, consider a ball pit:
    http://en.wikipedia.org/wiki/Ball_pit
     
    Last edited: Sep 16, 2014
  6. Sep 16, 2014 #5
    This is all related to Pascal's principle which roughly says that a fluid under pressure transmits force equally in all directions. Here is a link on Pascal's principle: http://en.wikipedia.org/wiki/Pascal's_law

    Chet
     
  7. Sep 16, 2014 #6
    Thx all for answers. Could any of you explain plz in simple words what causes particles of fluid or gas to expand (move) when they are under pressure (what force cause it)?
     
  8. Sep 16, 2014 #7

    Nugatory

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    They aren't expanding. What makes a fluid a fluid (and different from a gas) is that no matter how hard you squeeze a fluid, its volume doesn't change.

    Toothpaste is a fluid (a rather thick fluid, but still a fluid). So when I squeeze the sides of the tube and reduce the volume of the tube, the volume of the toothpaste stays the same. The only way that can happen is if some of the toothpaste leaves the tube because there's less room for it in the tube.

    You could try putting your thumb over the mouth of the tube, holding it closed while you squeeze the tube from the sides. You will feel the toothpaste in the tube pushing against your thumb. That's pressure caused by your thumb holding the toothpaste in the neck in place; as long as that toothpaste can't move nothing can leave the tube so the volume of the tube can't change; . The harder you squeeze the tube, the greater the pressure - the only way the sides of the tube can move inwards reducing the volume of the tube is if some of the toothpaste in the tube forces your thumb aside so that it can escape.
     
  9. Sep 17, 2014 #8
    I understand that volume must stay the same but I can't understand why. If I imagine toothpaste inside the tube as a group of atoms. When I move some of them ( my hand squeezing) all the other atoms are moving as well in all directions (pressure). Is that Electromagnetic force or something that causes these atoms to move ?
     
  10. Sep 17, 2014 #9

    A.T.

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    Yes the forces between atoms are electromagnetic.
     
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