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

[flatmaster]

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?

 

[mgb_phys]

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.

 

[rcgldr]

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.

 

[flatmaster]

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.

 

[sam_p_r]

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.