Frictional force nonrelated to N

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Discussion Overview

The discussion revolves around the nature of frictional forces, particularly in scenarios where the normal force (N) is effectively zero. Participants explore the case of a cylinder falling inside a pipe, examining how friction can still manifest despite the absence of a traditional normal force, and the implications of air compression in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how friction can occur when the normal force is zero, using the example of a cylinder falling inside a pipe where air cannot escape.
  • Another participant challenges the notion of heat production without contact, suggesting that heat generation implies some form of interaction with the pipe.
  • It is proposed that air compression beneath the cylinder could contribute to heating, even in the absence of a normal force.
  • Some participants mention the possibility of adhesive forces between the cylinder and the pipe wall, despite the surfaces being ideally smooth.
  • A later reply clarifies that while air cannot pass between the cylinder and the pipe, the compressed air underneath still exerts pressure on the cylinder.
  • There is a suggestion to consider the scenario in a vacuum to focus solely on the interaction with the wall.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms of friction in the absence of a normal force, with no consensus reached on how to account for the observed effects.

Contextual Notes

The discussion highlights assumptions regarding the behavior of air and friction in confined spaces, as well as the conditions under which adhesive forces may arise. The implications of these factors remain unresolved.

Breston
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The classical approximated formula describing frictional force is [itex]f = N\mu[/itex], directed oppositely to the motion direction.
What about that kind of friction that arises when N is 0? How can I account of that?
For example, suppose I let a cylinder fall inside a pipe nearly the same size, so that air cannot slip through the cylinder and the pipe. The motion is perfectly vertical (parallel to gravitational force). The cylinder falls slowly than it would otherwise, and produces heat on the surface of the pipe, displaying evident friction. Still, no vector N exists since the cylinder is falling and does not stand on a surface.
 
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Breston said:
The cylinder falls slowly than it would otherwise, and produces heat on the surface of the pipe, displaying evident friction. Still, no vector N exists since the cylinder is falling and does not stand on a surface.

You are saying that it would produce heat so how it would have produced heat if it had not contacted to the pipe? You can imagine this motion such as an object being pushed on a frictional surface.
 
Breston said:
For example, suppose I let a cylinder fall inside a pipe nearly the same size, so that air cannot slip through the cylinder and the pipe. The motion is perfectly vertical (parallel to gravitational force). The cylinder falls slowly than it would otherwise, and produces heat on the surface of the pipe, displaying evident friction. Still, no vector N exists since the cylinder is falling and does not stand on a surface.
If you have air being compressed by the cylinder, the air heats up. As for the wall, even with zero normal force, and ideally smooth surfaces you can have adhesive forces between wall and cylinder:
http://en.wikipedia.org/wiki/Adhesion
 
A.T. said:
If you have air being compressed by the cylinder, the air heats up. As for the wall, even with zero normal force, and ideally smooth surfaces you can have adhesive forces between wall and cylinder:
http://en.wikipedia.org/wiki/Adhesion

It is supposed that we do not let air get into the pipe so there could be found nothing in between to have made it heated.
 
PaulDirac said:
It is supposed that we do not let air get into the pipe
As I understand it, the air cannot pass between cylinder and pipe. But the air under the cylinder is still being compressed, and exerts pressure on the cylinder bottom.

If the OP is mainly interested in the interaction with the wall, then let's assume it's all in vacuum.
 

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