Why won't pressure on an irregular shape make it move?

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Discussion Overview

The discussion revolves around the question of why an object with an irregular shape does not move when subjected to pressure differences, particularly in fluids. Participants explore theoretical and practical implications of pressure on various shapes, including wedges and cones, and how these shapes interact with fluid dynamics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether an object with one side having twice the area of another would experience a net force causing movement due to pressure differences.
  • Another participant challenges the notion of having more cross-sectional area on one side than the other, emphasizing that pressure acts on all exposed surfaces, including sloped sides.
  • A wedge and a cone are proposed as examples, but participants argue that the pressure on sloped surfaces balances out the forces.
  • A hypothetical scenario involving an air-filled PVC pipe is presented to illustrate how the orientation of an object affects its movement in water.
  • Further exploration of shape dynamics is introduced with an elongated equilateral triangle, suggesting that different shapes can lead to varying rotational behaviors in fluid environments.
  • A creative example involving a basketball with a wedge-shaped attachment is discussed, proposing that such a configuration could allow for both vertical and lateral movement due to buoyancy and pressure dynamics.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are competing views on how pressure acts on irregular shapes and whether these shapes can experience net movement in a fluid. The discussion remains unresolved with various hypotheses presented.

Contextual Notes

Participants express uncertainty regarding the effects of pressure on different shapes and the conditions under which movement occurs. The discussion includes assumptions about fluid dynamics and the behavior of irregular shapes that are not fully explored.

Who May Find This Useful

This discussion may be of interest to those studying fluid dynamics, physics of buoyancy, or anyone curious about the interactions between pressure and irregular shapes in various environments.

Idoubt
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This is a very simple doubt, since pressure is force per unit area, if I place an object with say one side twice the area of the other underwater ( or even in the atmosphere ) wouldn't there be twice as much force on one side? ( the one with larger area ) and shouldn't that make the object move in the direction of that force?
 
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I would love to know what shape you have that has more cross sectional area on one side than the other (as force is directional, cross sectional area is in essence what matters).
 
lets say a wedge, with one edge a bit flat
 
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or imagine a cone with the top part chopped off and imagine the hole to be solid.
 
A wedge and a cone don't have more cross sectional area on one side than the other...

The sloped side is still exposed to the pressure!

Draw yourself a diagram: use a right triangle triangle sitting on the floor of a tank. Pressure acts perpendicular to each surface, so calculate the magnitude and direction of the forces on the sloped and vertical sides...then calculate the horizontal component of the force on the sloped side...
 
As Russ said, the sloped region is still subject to the pressure. Just because it isn't perfectly parallel to the opposite side doesn't mean that the net force is unbalanced. The exercise with the right triangle that Russ mentions is a good way to show this.
 
I don't know if this helps, but imagine an air-filled PVC pipe that is capped at each end.
Let's say that the pipe is 3-ft long with an inside diameter of 3-inches.

Go to your favorite swimming pool and force the pipe down as far as practical and then release.

Ver. 1) If held horizontally, then released, the air-filled PVC pipe will rise to the surface horizontally.

Ver. 2) If held vertically, then released, the pipe during ascent will tend to rotate until gaining a complete horizontal configuration. If the water depth is too shallow, the conversion does not significantly alter until complete ascent. If the water depth is deep, the conversion to a horizontal position occurs under water before reaching the surface.
 
Now, here is where it gets interesting:

Let's change the shape of the tube from being an elongated circle, to an elongated equilateral triangle.

Back to the swimming pool...

The end result of rotation to a horizontal position remains the same, but now a new factor is introduced, which causes an additional rotation around it's long axis, such to where one of the flat sides eventually points down, the end result being that 2-sides are now rising to and then above the surface.
 
With this information in hand, one might ask if it is possible to have both a vertical and lateral movement, at the same time, of a specifically shaped buoyant object underneath a body of water with no other internal/external forces applied.

The answer is yes. It's easy.

Imagine a basketball. Now, affix with strong glue a Plexiglas plate on the top of the basketball that is "wedge shape"... say, higher on the left side and sloping to flat on the right.

Now, introduce weighted materials such to weight balance the left-vs-right Plexiglas wedge.
Then, affix a reasonable weight to the center bottom of the basketball so that it doesn't flip over when submerged in the swimming pool yet can still rise from buoyancy.

There you have it!
During ascent, the ball will move upward by virtue of buoyancy, and laterally by virtue of wedge dynamics against the upper water.

It can be much easier than this, but this explains the potential.
 
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  • #10
thank you for the replies, I think I've got it now.
 

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