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Weight of a fluid when I put my finger in it

  1. Jan 21, 2013 #1
    Hi. I was asking myself the following question.

    What happens if I have a fluid at rest inside a container, and I introduce my finger in it without applying any force to the walls? Will the weight of the whole system change?

    My intuition says no (If I don't exert a force on the fluid this doesn't exert a force on the ground, and thus the normal force of the ground (weight) is still the same), but I don't really know how to explain it (I'm not really satisfied with the previous explanation). Can someone give me a hand?

    Thanks :)
  2. jcsd
  3. Jan 21, 2013 #2


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    You are exerting a downward pressure on the fluid. You have to push your finger in against the pressure of the fluid. That upward force is buoyancy. By Newton's third law the fluid experiences an equal and opposite downward force. The upward force on you and the downward force on the fluid are both computed by the density of the fluid times the displaced volume times the acceleration of gravity.

    A different way of arriving at the same conclusion is to notice that the height of the fluid will rise. That means more pressure on the bottom of the container. If it is resting on a scale, the scale will display a higher reading as a result.
  4. Jan 21, 2013 #3

    Doc Al

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    I assume you mean something like a beaker of water sitting on a scale and you put your finger in it without touching the walls? And your question is: does the scale reading change?
    What makes you think you don't exert a force on the fluid? Also, what happens to the level of the fluid in the container?
  5. Jan 21, 2013 #4


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    You've asked an interesting question. Why do you not arrange an experiment and try exactly what you describe? Find a good quality (probably digital readout) scale. Put a beaker full of liquid on it and record the weight. Then introduce your finger. Note any difference. You've already formed a hypothesis. This is clearly described as the "Scientific Method", which you could look up.

    Let us know the results!
  6. Jan 21, 2013 #5
    Is it possible to put your finger in the fluid without applying any force to the walls.

    You intuition will probably be wrong.Why not place a container on a scale with fluid in it and do what's required.
  7. Jan 21, 2013 #6
    Thanks for all the replies. I've understood what all of you meant about the buoyancy force.

    The weight of the container will increase as the total weight of water displaced by my finger, because the water is exerting a buoyant force on my finger and, by Newton's 2nd law, my finger exerts the same but opposite force on the liquid.

    Thanks so much :)
  8. Jan 21, 2013 #7

    Doc Al

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    Good! (But you mean Newton's 3rd law.)

    But do actually try it.
  9. Jan 21, 2013 #8


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    Try it with a small beaker, a large beaker and a very full Erlenmeyer flask (or any container that has a broad flat bottom and a narrow neck).

    Obviously the change in the height of the fluid in the various cases will be different. An analysis that looks at the resulting change in pressure on the bottom of the container will predict one thing. An analysis based on buoyancy will predict another.

    What does experiment say?

    Can you reconcile the apparent discrepancy between the two theoretical predictions?
  10. Jan 22, 2013 #9
    I'm interested and would love to conduct this experiment! I just don't have the equipment and resources to do it. :( Maybe next time.
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