Is the Coefficient of Static Friction Always Higher than Kinetic Friction?

Click For Summary

Discussion Overview

The discussion centers around the relationship between the coefficients of static and kinetic friction, specifically whether the coefficient of static friction is always higher than that of kinetic friction. Participants explore various materials and conditions under which this relationship may vary, including specific examples and hypothetical scenarios.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant states that the coefficient of static friction is usually higher than that of kinetic friction and questions under what conditions this might not hold true.
  • Another participant explains that static friction arises from bonds forming between surfaces, while kinetic friction involves both forming and breaking these bonds, suggesting that certain surface conditions could lead to kinetic friction being higher.
  • A different participant mentions that they are not aware of any solid-to-solid interface where kinetic friction is less than static friction, but notes that Teflon on Teflon exhibits similar coefficients for both types of friction.
  • One participant proposes that kinetic friction could be higher if the surfaces undergo chemical changes during sliding, emphasizing that this would complicate the comparison of coefficients.
  • A later reply reiterates the idea that if the chemical composition of the surfaces changes, the comparison of static and kinetic friction coefficients becomes less meaningful, as it involves different surface interactions.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between static and kinetic friction, with no consensus reached on whether kinetic friction can ever be higher than static friction under specific conditions. The discussion remains unresolved regarding the implications of chemical changes on friction coefficients.

Contextual Notes

Participants acknowledge that the discussion may depend on specific material properties and surface conditions, and that the definitions of static and kinetic friction may vary based on context.

flatmaster
Messages
497
Reaction score
2
One of our new labs states that the coefficient of static friction is USUALLY higher than the coefficient of kenitic friction. Is this true? If so, for what types of materials is the coefficient of static friction less than the coefficient of kenitic friction?
 
Physics news on Phys.org
Static friction is caused by bonds forming between the two surfaces - sliding friction is both making/breaking bonds as the surfaces move and partly breaking off high points on the surfaces as the rub against the other.

I can't immediately think of a case where sliding friction is higher but I could imagine for example where there is an oxide coating that forms on the surface reducing the bonds and this layer is rubbed off in sliding friction allowing surface-surface bonds to form.
 
I'm not aware of any solid to solid interface where dynamic friction is less than static friction, but teflon on teflon has about the same static and dynamic friction.

By injecting a fluid into ports on a rail, a slideway (do a web search) uses the fluid to increase drag associated with any movement. This creates a situation where kinetic friction + drag is higher than static, and one where the drag increases with speed. It's useful for smooth control of movements.
 
Ok, so you might say that the only way that kenitic friction could be higher is if the surfaces themselves change chemically as a result of the sliding? I'm talking about a single surface to surface contact. Not with any intermediate lubrication as Jeff Reid suggested.
 
flatmaster said:
Ok, so you might say that the only way that kenitic friction could be higher is if the surfaces themselves change chemically as a result of the sliding? I'm talking about a single surface to surface contact. Not with any intermediate lubrication as Jeff Reid suggested.

But if the chemical composition changes, then the question has no value because the coeffecient of friction is between the two surfaces, and a surface has changed. ie. all you would be saying is that the kinetic friction is higher between these two surfaces than the static friction between these two surfaces.

... like saying the kinetic coeff. between steel on steel is higher than the static coeff. between teflon on teflon.
 

Similar threads

Undergrad If frictional force in system is <= us*N then there is no slipping
  • · Replies 7 ·
Replies
7
Views
2K
Undergrad Why is rolling easier than sliding?
  • · Replies 71 ·
3
Replies
71
Views
11K
High School Friction and Newton's laws in car braking systems
  • · Replies 10 ·
Replies
10
Views
2K
High School Static friction needed for rolling without slipping
  • · Replies 4 ·
Replies
4
Views
2K
High School Are frictional forces action forces or reaction forces?
  • · Replies 14 ·
Replies
14
Views
2K
Undergrad I would like a better understanding of friction and hysteresis
  • · Replies 5 ·
Replies
5
Views
3K
High School Difference between static and kinetic friction
  • · Replies 10 ·
Replies
10
Views
3K
Undergrad Static rotational friction: does diameter matter?
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Coefficient of friction of various materials not listed in standard references
  • · Replies 7 ·
Replies
7
Views
4K
Undergrad Determine if the block will slide or flip over after contact
  • · Replies 8 ·
Replies
8
Views
2K