Understanding Buoyancy and Drag Forces in Fluids: Causes and Differences

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Discussion Overview

The discussion centers on understanding the concepts of buoyancy and drag forces experienced by objects in fluids. Participants explore the causes and differences between these two forces, addressing both theoretical and conceptual aspects.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant states that the buoyant force arises from the pressure differential across the top and bottom of an object, existing independently of motion.
  • Another participant explains that drag force is a motional force resulting from the object colliding with fluid molecules as it moves through the fluid.
  • A participant seeks clarification on the term "pressure differential," asking for an explanation of its meaning and causes.
  • One participant elaborates that pressure is defined as force per unit area and describes how the concentration of air molecules decreases with altitude, leading to a greater pressure on the lower part of the object compared to the upper part, which contributes to buoyancy.
  • Another participant questions whether drag force is influenced by the frequency of collisions with air molecules or the intensity of those collisions, indicating a need for further exploration of the mechanics of drag.

Areas of Agreement / Disagreement

Participants express differing views on the mechanics of drag force and its relationship to the motion of objects through fluids. The discussion remains unresolved regarding the specifics of how drag force operates compared to buoyancy.

Contextual Notes

Some assumptions about fluid dynamics and the definitions of pressure and drag are not fully explored, leaving room for further inquiry into the nuances of these forces.

Cheman
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Why does an object in a fluid experience both a buoyancy force and a drag force? How are they different and what causes these two different forces?
Thanks. :-)
 
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The buoyant force is caused by the pressure differential across the top and bottom of the object. It exists regardless of motion.

Drag force is purely a motional force.

Both are caused by the impacts of nearby fluid molecules.
 
Thanks, but what do you mean by "pressure differential across the top and bottom of the object"? Could you please explain what this means, what causes this, etc?
Thanks. :-)
 
Pressure is a measure of the force applied per unit of area. In the case of an body in the air, the pressure results from air molecules impinging the surface of the body at high speeds. Because the concentration of air molecules decreases as one rises from the Earth (notice that there is a high concentration near the surface but clearly no air in space, the fall off is continous), the pressure exerted on the lower portion of the body is greater than that exerted on the upper portion of the body (less air molecules impinging the upper surface at a higher altitude). This difference in pressure, or pressure differential, is what causes the bouyant force.

Air drag is a little different in that it results from the body colliding into a large number of air molecules as it travels through the air. The faster it travels, the more air molecules the body has to push out of its way in a unit of time, so there is greater resistance at higher speeds.
 
With drag, is it that the object is colliding with eg - air molecules more often or hitting them harder, hence experiancing a greater force?
 

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