Why Does a Cube Float When Dipped in Water?

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Discussion Overview

The discussion revolves around the phenomenon of buoyancy, specifically why a plastic cube floats when submerged in water. Participants explore the relationship between pressure, buoyant force, and the density of the cube, engaging in a conceptual examination of fluid mechanics.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that the cube floats due to its lower density compared to water, while questioning the application of Pascal's Law in this context.
  • Others clarify that pressure increases with depth, resulting in higher pressure at the bottom of the cube than at the top, which contributes to the buoyant force.
  • A participant challenges the idea that the pressure from the water above the cube is greater than the buoyant force from below, stating that this is incorrect and that the pressure on the top is indeed less than that on the bottom.
  • There is a discussion about whether the amount of water above affects the buoyant force, with some arguing that it does not matter, while others express confusion about this point.
  • One participant emphasizes that the net force acting on the cube is upwards due to the greater pressure at the bottom compared to the top.

Areas of Agreement / Disagreement

Participants express differing views on the role of pressure from the water above and below the cube, leading to a lack of consensus on how these forces interact to produce buoyancy. Some participants agree on the importance of pressure differences, while others remain uncertain about the implications of these differences.

Contextual Notes

The discussion reveals limitations in understanding the relationship between pressure and buoyancy, particularly regarding the assumptions about water communication and the effects of depth on pressure. Some mathematical steps and definitions remain unresolved.

Who May Find This Useful

This discussion may be useful for individuals interested in fluid mechanics, physics students exploring buoyancy concepts, or anyone seeking clarification on the principles of pressure in fluids.

Pranav
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Imagine a container containing water up to 100cm of its height then I dip a cube of plastic in the water on depth of 75cm. We all know it will float because of its less density, but if we go with Pascal Law, The pressure/force applied by the water above the cube is more than than the buoyant force/pressure applied by the water below, and if the buoyant force is less then the cube should not float. But when did the same at my home in a bucket the cube came to surface. Why??
 
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Pranav said:
Imagine a container containing water up to 100cm of its height then I dip a cube of plastic in the water on depth of 75cm. We all know it will float because of its less density, but if we go with Pascal Law, The pressure/force applied by the water above the cube is more than than the buoyant force/pressure applied by the water below, and if the buoyant force is less then the cube should not float. But when did the same at my home in a bucket the cube came to surface. Why??

Pressure increases with the depth of the water, hence the pressure at the bottom of the cube is higher than at its top. If the difference of these two pressure forces is larger than the weight of the cube, it floats. You must not compare the "height" of the water below and above the cube, but the pressure vs. the depth.
 
Pranav said:
The pressure/force applied by the water above the cube is more than than the buoyant force/pressure applied by the water below,
This is incorrect. I am not sure where you got this idea, but it is exactly backwards. The pressure on the top is less than the pressure on the bottom. This is the cause of the buoyant force
 
Dale said:
The pressure on the top is less than the pressure on the bottom. This is the cause of the buoyant force
Well, I know that sir but if their is more water above than below then, the water above push the box down. Right??
 
stockzahn said:
Pressure increases with the depth of the water, hence the pressure at the bottom of the cube is higher than at its top. If the difference of these two pressure forces is larger than the weight of the cube, it floats. You must not compare the "height" of the water below and above the cube, but the pressure vs. the depth.
If their is more water above than below then, the water above push the box down as it will apply more downward force than the upward force . Right??
 
Pranav said:
If their is more water above than below then, the water above push the box down as it will apply more downward force than the upward force . Right??

Only if the water above and below the cube is not "communicating" (e.g. in two separated compartments). The weight of the water above the cube acts around the cube on the water below the cube and increases the pressure. And why shouldn't it? Otherwise that would mean that the weight force of all the water above the cube is lying on the top of the cube.
 
Pranav said:
Well, I know that sir but if their is more water above than below then, the water above push the box down. Right??
No. The amount of water below is completely irrelevant.
 
stockzahn said:
Only if the water above and below the cube is not "communicating" (e.g. in two separated compartments). The weight of the water above the cube acts around the cube on the water below the cube and increases the pressure. And why shouldn't it? Otherwise that would mean that the weight force of all the water above the cube is lying on the top of the cube.
Sorry but i didn't understand
 
@Pranav all that matters is the pressure. The pressure is determined by the depth of water above. The amount of water below does not matter. The pressure increases as you go down, therefore the bottom is always subjected to a greater pressure than the top.
 
  • #10
Dale said:
@Pranav all that matters is the pressure. The pressure is determined by the depth of water above. The amount of water below does not matter. The pressure increases as you go down, therefore the bottom is always subjected to a greater pressure than the top.
And more the pressure more you will be pushed down??
 
  • #11
Pranav said:
Sorry but i didn't understand
 

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  • #12
Pranav said:
And more the pressure more you will be pushed down??
No, you are pushed in a direction normal to the surface. That is down on the top, up on the bottom, and inwards on the sides.

So the upwards force on the bottom is greater than the downwards force on the top. The net force is therefore upwards (buoyant)
 
Last edited:
  • #13
Thanks you everyone.
 
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  • #14
Now it's clear to me.
 

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