Drag Force: Mass & Why It Doesn't Affect Object

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Discussion Overview

The discussion centers on the nature of drag force and its relationship with the mass of an object. Participants explore theoretical and conceptual aspects of drag force, including its dependence on various factors such as shape, density, and the dynamics of air molecules.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question why drag force does not depend on the mass of the object.
  • Others inquire about the factors that drag force does depend on, suggesting it relates to the interaction between air molecules and the object.
  • One participant proposes that if drag force is viewed as a reaction force during collisions with air molecules, then it should depend on the momentum of both the air molecules and the object.
  • Another participant asserts that drag force is determined by the shape of the object, regardless of whether it is hollow or solid.
  • Some argue that if the object's mass changes while its velocity remains constant, the drag force must also change, implying a dependence on mass.
  • The drag equation is referenced multiple times, indicating that it involves factors such as the object's density, cross-sectional area, and shape, but one participant clarifies that it is actually the air density that is relevant.
  • There is a discussion about the limitations of the drag equation, noting that it is empirical and does not encompass all physical factors contributing to drag, such as viscosity and flow characteristics.
  • One participant emphasizes that in none of the discussed cases does the mass or density of the object influence the drag force.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between drag force and mass, with no consensus reached on whether mass influences drag force. Multiple competing perspectives on the factors affecting drag force remain present in the discussion.

Contextual Notes

The discussion highlights the complexity of drag force and its dependence on various physical parameters, with some assumptions and definitions remaining unresolved.

ahmed emad
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Why drag force doesn't Depend on the mass of the Object?
 
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Well, what does drag force depend on?
 
if the drag force is the reaction force on the elastic collision between air molecules and the object , so it have to depend on the momentum of the air molecules and object momentum, is that right?
 
Drag force (and all aerodynamic forces) are the forces caused by the air flowing around an object. Only the shape of the object matters. It doesn't matter if the object is hollow or filled with lead.
 
okay ,but if you see it as a collision and the air molecule mass is constant and its initial velocity is also constant, and the object velocity is also constant but its mass is changing so the reaction force will change , so drag force must depend mass.
 
The drag equation suggests it's the object's density, cross-sectional area, and shape that affects the force acting on it by the air.
 
A Lazy Shisno said:
The drag equation suggests it's the object's density, cross-sectional area, and shape that affects the force acting on it by the air.
That's the air density, not the object's density.
 
FactChecker said:
That's the air density, not the object's density.
Sorry, yes, you're right. I'm not sure how I forgot that.
 
A Lazy Shisno said:
The drag equation suggests it's the object's density, cross-sectional area, and shape that affects the force acting on it by the air.

The "drag equation" also is not based upon first principles and does not capture all of the relevant physics contributing to drag. It's just an empirical relationship between the drag force, area, and the dynamic pressure (##\rho V^2/2##). The drag coefficient relates those quantities across a wide range of conditions, but tells you nothing about what actually contributes to the drag. Viscosity, ratio of specific heats, and Mach number can also all play a role, for example. So can the laminar-turbulent state of the boundary layer, the surface quality, and in extremely high-speed flows, chemical reactions.

The important thing is that in none of those cases does the mass (or density) of the object play a role.
 
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