How does kinetic friction occur between two moving bodies?

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Discussion Overview

The discussion revolves around the phenomenon of kinetic friction between two moving bodies, exploring the molecular and macroscopic interactions that contribute to this force. Participants examine the mechanics of friction, its direction relative to motion, and the factors influencing its magnitude, including the normal force.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants inquire about the molecular mechanisms behind kinetic friction, suggesting that surface irregularities play a significant role.
  • One participant illustrates the concept of friction using a metaphor involving rough surfaces, emphasizing the resistance encountered compared to smooth surfaces.
  • There is a question raised about whether kinetic friction always acts in the opposite direction of motion, with some asserting that it does, while others argue that it depends on the context of relative motion.
  • Participants discuss scenarios involving two blocks, where friction may not seem to oppose motion, leading to further clarification that friction always opposes relative motion between surfaces.
  • Another topic of discussion is the proportionality of friction to the normal force, with some suggesting it is an engineering approximation rather than a fundamental principle.
  • Concerns are raised about the reliability of models explaining friction, with participants expressing uncertainty about their correctness and applicability in various contexts.

Areas of Agreement / Disagreement

Participants generally agree that kinetic friction opposes relative motion, but there is contention regarding specific scenarios and interpretations of this principle. The discussion on the proportionality of friction to normal force also reveals differing views on its fundamental nature.

Contextual Notes

Some participants note that the discussion involves assumptions about surface characteristics and the definitions of friction types, which may not be universally applicable. There are also references to specific models that may not capture the full complexity of frictional interactions.

Who May Find This Useful

This discussion may be useful for students and enthusiasts of physics, particularly those interested in the mechanics of friction, its underlying principles, and the complexities involved in its study.

  • #31
parshyaa said:
A body slipping on a rough horizontal plane moves with a deceleration of 4m/s^2.

In this case we are taking kinetic friction as the reason for accelaration
But kinetic friction resist the relative motion, then what is wrong, if in this case its helping to accelarating

You said opposite things. First you say it is decelerating (slowing down; eventually to stop). Then you say it accelerating (speeding up). You are confusing yourself.
 
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  • #32
parshyaa said:
In this case we are taking kinetic friction as the reason for accelaration
But kinetic friction resist the relative motion, then what is wrong, if in this case its helping to accelarating

Kinetic friction resists motion, that is correct.
The velocity is positive so Fkinetic friction is negative and so the acceleration is negative (=deceleration).

PS: Fkinetic friction is not a constant force. As soon as the velocity reaches zero Fkinetic friction becomes zero and is replaced by static friction. So Fkinetic friction cannot make the block accelerate in the opposite direction.
 
  • #33
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IMG_20171130_212328000_HDR.jpg

If this fbd is correct then fk is in the direction of accelaration this is why i said its towards acceleration or more precisely its the reason for accelaration due to fbd, but why kinetic friction is helping to acc. In this case
 

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  • #34
parshyaa said:
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View attachment 215847
If this fbd is correct then fk is in the direction of accelaration this is why i said its towards acceleration or more precisely its the reason for accelaration due to fbd, but why kinetic friction is helping to acc. In this case
The frictional force and the associated acceleration are both in the direction opposite to the relative motion. What is the problem?
 
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  • #35
jbriggs444 said:
The frictional force and the associated acceleration are both in the direction opposite to the relative motion. What is the problem?
Yes you are right, my confusion has gone now

I could solve problem containing two or three surfaces but solving questions containing only two surfaces sometimes makes me confuse but now i am cleared
Thanks @jbriggs444 and @anorlunda
 

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