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

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SUMMARY

The coefficient of static friction is generally higher than the coefficient of kinetic friction, particularly in solid-to-solid interactions. However, exceptions exist, such as Teflon on Teflon, where both coefficients are similar. Additionally, scenarios involving chemical changes to surfaces during sliding can result in kinetic friction exceeding static friction, particularly when surface bonds are altered. The introduction of fluids in systems, such as slideways, can also create conditions where kinetic friction plus drag surpasses static friction.

PREREQUISITES
  • Understanding of friction types: static and kinetic friction
  • Knowledge of material properties and surface interactions
  • Familiarity with chemical changes in materials during contact
  • Basic principles of fluid dynamics in mechanical systems
NEXT STEPS
  • Research the properties of Teflon and its friction coefficients
  • Investigate the effects of surface coatings on friction behavior
  • Explore the mechanics of slideways and fluid injection systems
  • Study the chemical interactions at solid-to-solid interfaces during sliding
USEFUL FOR

Mechanical engineers, materials scientists, and anyone interested in the dynamics of friction in engineering applications.

flatmaster
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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?
 
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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.
 

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