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I Pressure and Force

  1. Jul 24, 2017 #1
    A container of water is sitting on a scale. I dip my finger into the water without touching the sides or bottom of the container. Is it true that the force exerted on the scale by the container will decrease because the finger creates a downward force?
  2. jcsd
  3. Jul 24, 2017 #2
    I meant that the force will increase.
  4. Jul 24, 2017 #3


    Staff: Mentor

    Are you familiar with Newton's laws?

    You can treat the water+container as a single object and draw a very simple free body diagram for it.
  5. Jul 24, 2017 #4
    Yes I am. Looking at newtons law, I feel that the force exerted will increase. I just wanted some confirmation.
  6. Jul 24, 2017 #5


    Staff: Mentor

    Yes, you have applied Newtons laws to reach the correct conclusion
  7. Jul 25, 2017 #6
    so, liquids are interesting. what happens just before your finger breaks the surface tension of the water? look at this same problem as the viscosity of the liquid thickens.

    my hypothesis is, using water on a accurate micro-grams scale, the scale reading goes up from no-touch to the point where your finger breaks the surface tension, and then slightly down as the water wicks up your finger after breaking surface tension.
  8. Jul 25, 2017 #7


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    Science Advisor

    If surface tension is something that you must break then your finger must be hydrophobic. Water should not wick up a hydrophobic finger.

    Experiment indicates that fingers are hydrophillic. There is no surface tension to break. Water wicks up from the moment contact is made. However, my test (in the sink in a handy rest room) was not sensitive enough to determine if there was an attraction even before contact was made.
  9. Jul 25, 2017 #8
    i dunno, oily finger perhaps, add some chapstick. i guess its one of the unknowns, or, perhaps multiple answers?
  10. Jul 25, 2017 #9


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    Your finger will be pressed up by a force equal to the of the weight of the amount of water it displaces, and every force must be balanced by an equal and opposite force, so a sensitive scale under the container will register this added force.
  11. Jul 26, 2017 #10
    Suppose you dipped your finger into the water and, at the same time, removed from the container the exact same volume of water that your finger displaces below the surface (so that the level of the surface is unchanged). Neglecting the tiny effect of surface tension, do you think the reading of the scale would increase, decrease, or remain the same.
  12. Jul 26, 2017 #11
    I placed a cup filled with water on a scale. It read 9.1 oz. I stuck my finger into the water and the reading went up to 9.4 oz. Case closed.
  13. Jul 26, 2017 #12
    The surrounding water doesn't know that it is your finger that is filling the displaced space. It thinks that there is still water present there, or, more precisely, it develops a hydrostatic pressure distribution that is the same as if water was present in the submerged space occupied by your finger. This includes the pressure at the very base of the container, where the pressure is now higher. Therefore, the reading on the scale will increase by the weight of a volume of water equal to the submerged volume of your finger.
  14. Jul 26, 2017 #13
    The submerged volume of your finger must have been about 0.3 fl oz. See my post #12.
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