Weight of a fluid when I put my finger in it

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SUMMARY

The discussion centers on the effect of introducing a finger into a fluid at rest within a container and its impact on the overall weight of the system. Participants clarify that inserting a finger displaces fluid, resulting in an upward buoyant force that corresponds to a downward force on the fluid, as described by Newton's third law. Consequently, the weight measured by a scale under the container will increase due to the additional weight of the displaced fluid. The experiment suggested involves using a digital scale to measure the weight before and after the finger is introduced into the fluid.

PREREQUISITES
  • Understanding of buoyancy and Archimedes' principle
  • Familiarity with Newton's laws of motion, particularly Newton's third law
  • Basic knowledge of fluid mechanics
  • Experience with scientific experimentation and measurement techniques
NEXT STEPS
  • Conduct an experiment using a digital scale and a beaker of water to observe weight changes
  • Research Archimedes' principle and its applications in fluid mechanics
  • Explore Newton's laws of motion in detail, focusing on practical examples
  • Investigate the relationship between fluid pressure and fluid displacement
USEFUL FOR

This discussion is beneficial for physics students, educators, and anyone interested in understanding fluid dynamics and the principles of buoyancy and force interactions in fluids.

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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