# Weight of a fluid when I put my finger in it

• Pepealej
In summary, you should try putting a beaker of water on a scale and see if the weight changes when you add your finger.
Pepealej
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 :)

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.

## 1. What is the weight of a fluid when I put my finger in it?

The weight of a fluid when you put your finger in it will depend on the density of the fluid and the volume of the fluid displaced by your finger. This can be calculated using the formula W = ρVg, where W is the weight, ρ is the density, V is the volume, and g is the acceleration due to gravity.

## 2. Does the weight of a fluid change when I put my finger in it?

Yes, the weight of a fluid will increase when you put your finger in it because your finger displaces a certain volume of the fluid, which adds to the total weight of the fluid. However, the weight difference may be very small and difficult to measure.

## 3. Will the weight of a fluid change if I use a different finger?

No, the weight of a fluid will not change if you use a different finger. This is because the volume of fluid displaced by your finger will remain the same, as long as your fingers have the same size and shape.

## 4. How does the weight of a fluid change when I put my finger in at different depths?

The weight of a fluid will increase as you put your finger in at different depths because the deeper you go, the more volume of fluid your finger displaces. This means that more weight will be added to the total weight of the fluid.

## 5. Why does the weight of a fluid change when I put my finger in it?

The weight of a fluid changes when you put your finger in it because your finger displaces a certain amount of fluid, which adds to the total weight of the fluid. This is due to the principle of buoyancy, which states that the weight of an object is equal to the weight of the fluid it displaces.

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