Weight of a fluid when I put my finger in it

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Discussion Overview

The discussion revolves around the question of whether introducing a finger into a fluid at rest in a container changes the weight of the system, particularly in the context of buoyancy and pressure. Participants explore theoretical implications and suggest experimental approaches to investigate the question.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant questions whether it is possible to introduce a finger into the fluid without applying force to the walls of the container.
  • Another participant argues that introducing the finger exerts a downward pressure on the fluid, leading to an upward buoyant force, which results in an increase in the weight measured by a scale.
  • Some participants suggest conducting an experiment to measure the weight change when a finger is introduced into the fluid, emphasizing the importance of empirical evidence.
  • There is a discussion about the relationship between buoyancy and pressure, with some participants noting that different theoretical analyses may yield different predictions regarding the weight change.
  • One participant acknowledges understanding the concept of buoyancy and its implications for the weight of the system.

Areas of Agreement / Disagreement

Participants express differing views on whether the weight of the system changes when a finger is introduced into the fluid. While some support the idea that it does change due to buoyancy, others question the initial assumptions regarding force application. The discussion remains unresolved with multiple competing views.

Contextual Notes

Participants note that the change in fluid height and pressure may vary depending on the container used, suggesting that the results could differ based on experimental conditions. There is also mention of potential discrepancies between theoretical predictions based on buoyancy and pressure analysis.

Who May Find This Useful

This discussion may be of interest to individuals exploring concepts in fluid mechanics, buoyancy, and experimental physics, as well as those interested in the scientific method and empirical investigation.

Pepealej
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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 :)
 
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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.
 
Pepealej said:
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?
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?
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?
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?
 
Pepealej,

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!
Cheers,
Bobbywhy
 
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?
Is it possible to put your finger in the fluid without applying any force to the walls.

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?
You intuition will probably be wrong.Why not place a container on a scale with fluid in it and do what's required.
 
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 :)
 
Pepealej said:
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.
Good! (But you mean Newton's 3rd law.)

But do actually try it.
 
Doc Al said:
But do actually try it.

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?
 
Bobbywhy said:
Pepealej,

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!
Cheers,
Bobbywhy

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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