Friction

[Mokele]

Ok, potentially dumb question: Why doesn't friction involve surface area? Is the friction force on a 10kg object really the same whether the object's contact area is a square centimeter or a square meter?

 

Mokele

[ecoli]

yeah I think so...

 

After all, even if you have less surface area, you're going to have more weight behind it... or should that not matter?

[RyanJ]

Ok, potentially dumb question: Why doesn't friction involve surface area? Is the friction force on a 10kg object really the same whether the object's contact area is a square centimeter or a square meter?

 

Mokele

 

Actually I'm pretty sure that surface area will directly affect the frictional force. If you have a rough surface (the rough surface in effect increasing the area on which the resistive force can act) even with the same mass we have more friction on the rough surface so does that indicate a link with surface area?

 

Is that idea is correct it would mean that the larger the affecting surface area the larger the frictional force, this does not actually depend on the mass that is pushing down on it which is assumed to be constant in this case (we would need to use the same mass only changing the surface area).

 

Maybe one the the physics guys can answer this more correctly, just me idea Looking at a relationship between friction and pressure here would also be interesting.

[swansont]

A lot of the physics treatment of friction is an approximation, but it's true the standard equation doesn't involve surface area. If the total area is greater, each unit area exerts less force (i.e. the pressure is smaller), and contributes less friction. In the approximation used, that exactly cancels with the greater surface area, and that's observed to hold in many cases.

[J.C.MacSwell]

A lot of the physics treatment of friction is an approximation, but it's true the standard equation doesn't involve surface area. If the total area is greater, each unit area exerts less force (i.e. the pressure is smaller), and contributes less friction. In the approximation used, that exactly cancels with the greater surface area, and that's observed to hold in many cases.

 

And of course the equation still holds, the coefficient may change, but usually not by much for the reasons you mention.

[ecoli]

A lot of the physics treatment of friction is an approximation, but it's true the standard equation doesn't involve surface area. If the total area is greater, each unit area exerts less force (i.e. the pressure is smaller), and contributes less friction. In the approximation used, that exactly cancels with the greater surface area, and that's observed to hold in many cases.

 

Yeah, that's what I was trying to say.

[gcol]

I wonder if people confuse friction and "sticktion"? By sticktion I mean the initial force (not inertia) required to initiate movement between two surfaces, and friction the ongoing resistance to continued motion. Take two rough surfaces with equal roughness. At rest, the roughness will tend to interlock. Once moving (having overcome sticktion) the rough surfaces will not have time to interlock, the surfaces roughness tending to bounce over itself. In fact, with a given combination and type of roughness, there may be an optimum "unsticking" velocity.

 

Sticktion is an engineering fact. Sometimes it is called rolling resistance as when a cartwheel requires a lot of effort to roll it out of a rut, but once moving it will tend to bounce over other ruts with no more real friction than if it were on a smooth road.

[ecoli]

I wonder if people confuse friction and "sticktion"? By sticktion I mean the initial force (not inertia) required to initiate movement between two surfaces, and friction the ongoing resistance to continued motion. Take two rough surfaces with equal roughness. At rest, the roughness will tend to interlock. Once moving (having overcome sticktion) the rough surfaces will not have time to interlock, the surfaces roughness tending to bounce over itself. In fact, with a given combination and type of roughness, there may be an optimum "unsticking" velocity.

 

Sticktion is an engineering fact. Sometimes it is called rolling resistance as when a cartwheel requires a lot of effort to roll it out of a rut, but once moving it will tend to bounce over other ruts with no more real friction than if it were on a smooth road.

 

Isn't that called limiting friction?

[EvoN1020v]

I wonder if people confuse friction and "sticktion"? By sticktion I mean the initial force (not inertia) required to initiate movement between two surfaces, and friction the ongoing resistance to continued motion. Take two rough surfaces with equal roughness. At rest, the roughness will tend to interlock. Once moving (having overcome sticktion) the rough surfaces will not have time to interlock, the surfaces roughness tending to bounce over itself. In fact, with a given combination and type of roughness, there may be an optimum "unsticking" velocity.

 

Sticktion is an engineering fact. Sometimes it is called rolling resistance as when a cartwheel requires a lot of effort to roll it out of a rut, but once moving it will tend to bounce over other ruts with no more real friction than if it were on a smooth road.

 

I think you're talking about static friction. It's a FACT that static friction is usally higher than kinetic friction so the object won't slip at stationary. When the object overcomes the static friction, it will experience a "stick slip" during the kinetic friction movement.

 

[gcol]

Isn't that called limiting friction?

 

I spelled it wrong. It is now called stiction, and the term is commonly accepted as having been "invented" in the 90's in connection with hard disk drives, (breaking a "goo" bond between head and platter) though it was known to me long before that in connection with the idea I am floating. Don't know about limiting friction, seems more sensible to call it initial friction.

 

How about the case of two surfaces having a sharp crystalline structure of equal topography, thus interlocking. Much force required to initiate movement, but once moving the surfaces will bounce over each other, no? Seems to me the greater the weight acting on the two surfaces, the greater initial mechanical unlocking force required, as distinct from ongoing movement friction.

 

I am really asking out of curiousity, not giving an answer. I think there is more to friction than meets the eye.

[swansont]

That may be an engineering term. As EvoN1020v has demonstrated, in physics the terms static and kinetic friction are often/usually used for the two cases.

 

There is more than meets the eye. The "cartoon" explanation of bumps that hit each other is almost certainly wrong. As is the notion that "smooth" surfaces have little friction.

[gcol]

And surely only two surfaces that do not deform under pressure from each other can have "pure" friction? If a small area of dense material presses on a softer, and creates a dent or well, surface friction is not the only counter-movement force to be considered? (This goes back to the original question?)

 

A cartoon explanation? Do you mean like rubber sheets and bowling balls to explain gravity? There are more cartoon explanations in physics than there are episodes of Mickey Mouse. What is wrong with another one?

[swansont]

Cartoon explanations aren't inherently bad, usually just watered-down or analogies; I merely noted that the one used for friction is wrong, which is bad.

[gcol]

I'm done now, in that case.

[s pepperchin]

The friction is dependent on the normal force and the force vectors. I always thought that that would explain why the surface area is not a part of the equation.

[5614]

The friction is dependent on the normal force and the force vectors. I always thought that that would explain why the surface area is not a part of the equation.

No, read what swansont wrote, below. I can't word it better than that.

 

If the total area is greater, each unit area exerts less force (i.e. the pressure is smaller), and contributes less friction. In the approximation used, that exactly cancels with the greater surface area, and that's observed to hold in many cases.