Why is kinetic friction constant?

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Discussion Overview

The discussion revolves around the nature of kinetic friction and its constancy compared to air friction, particularly focusing on the differences in how these forces behave at varying velocities. Participants explore both mathematical and conceptual aspects of friction in different contexts, including fluid dynamics.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants express confusion about why air friction increases quadratically with velocity while kinetic friction is considered constant.
  • One participant notes that the formula for kinetic friction is empirically derived and only approximately true, suggesting that it does not hold universally.
  • Another participant explains the distinction between linear and quadratic drag, indicating that linear drag is more relevant for small objects while quadratic drag dominates for larger objects.
  • Concerns are raised about the mathematical explanations provided, with some participants indicating a preference for a more conceptual understanding of the topic.
  • Links to external resources are shared to provide additional information on friction.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the conceptual understanding of kinetic friction versus air friction, with some favoring mathematical explanations while others seek a more intuitive grasp of the concepts.

Contextual Notes

Participants highlight the complexity of fluid dynamics and the varying behavior of air friction at different speeds, indicating that assumptions about the constancy of kinetic friction may not apply universally.

Skhandelwal
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I don't get it, first of all, why does air friction quadruples as velocity doubles? Second, if that air friction works in that way, why does kinetic work in a constant manner?
 
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Skhandelwal said:
I don't get it, first of all, why does air friction quadruples as velocity doubles? Second, if that air friction works in that way, why does kinetic work in a constant manner?
The short answer is that it doesn't, exactly. The formula for kinetic friction is empirically derived and is only approximately true.

Air friction does not always vary as the square of the speed. At slower speeds it is proportional to speed. At higher speeds it is proportional to higher orders of the velocity. Fluid dynamics is very complicated and has to take into consideration many different things that go on when an object moves through a fluid.

AM
 
approximately... there is linear drag and quadratic drag. linear drag dominates for small objects (example: a piece of dust), linear drag is caused by aerodynamic flow passing the object...

for big objects, quadratic drag dominates. You can think of it this way, when an big object is moving, it acclerates the air in front of it. Let's say the objects is moving at v and the air is stationary relative to it. for time dt, the object moves a distance of v*dt, so that the number of molecules it hits is around v*dt*(area), the momentum transferred is around m*v so that the the change in momentum per dt ~ m*v*v*dt*(area) ~ v^2. since force is change in momentum per time, the drag goes quadratically...

my explanation is very very VERY rough, do not take it too literally.
 
Last edited:
all of you are explaning it mathematically, I have problem understanding it conceptually.
 
Anyone? Please?
 
Skhandelwal said:
all of you are explaning it mathematically, I have problem understanding it conceptually.
Physics is based upon mathematics.
 

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