Friction Between Different Materials

[Stan Pope]

I've been turning this about in my mind for a few days and haven't come up with a satisfactory answer ... new ideas will be appreciated.

Suppose that I have two flat surfaces, one hard (e.g. metal) and one soft (e.g. plastic).

Case 1:
The soft surface is flat +/- 1 micron; the hard surface is flat +/- 10 microns.

Case 2:
The soft surface is flat +/- 10 microns; the hard surface is flat +/- 1 micron.

Case 3:
Both surfaces are flat +/- 10 microns.

How will the sliding Cf's compare? How will the wear patterns compare?
(Identify assumed surface textures, e.g. ripple, sawtooth, etc)

[*5 J's*]

I've been turning this about in my mind for a few days and haven't come up with a satisfactory answer ... new ideas will be appreciated.

Suppose that I have two flat surfaces, one hard (e.g. metal) and one soft (e.g. plastic).

Case 1:
The soft surface is flat +/- 1 micron; the hard surface is flat +/- 10 microns.

Case 2:
The soft surface is flat +/- 10 microns; the hard surface is flat +/- 1 micron.

Case 3:
Both surfaces are flat +/- 10 microns.

How will the sliding Cf's compare? How will the wear patterns compare?
(Identify assumed surface textures, e.g. ripple, sawtooth, etc)

Case 2 is faster. Sorry, taking a night off from physics.

[FatSebastian]

How will the sliding Cf's compare?

According to this page, the coefficient of friction between two sliding materials may be governed by, among other things, the "surface roughness of the relative[ly] harder contact surface." If true, then Case 2 results in the lowest coefficient of friction.

[Stan Pope]

I've been turning this about in my mind for a few days and haven't come up with a satisfactory answer ... new ideas will be appreciated.

Suppose that I have two flat surfaces, one hard (e.g. metal) and one soft (e.g. plastic).

Case 1:
The soft surface is flat +/- 1 micron; the hard surface is flat +/- 10 microns.

Case 2:
The soft surface is flat +/- 10 microns; the hard surface is flat +/- 1 micron.

Case 3:
Both surfaces are flat +/- 10 microns.

How will the sliding Cf's compare? How will the wear patterns compare?
(Identify assumed surface textures, e.g. ripple, sawtooth, etc)

Case 2 is faster. Sorry, taking a night off from physics.

I think that I agree ... let's add one more case. How does this compare?

Case 4:
Both surfaces are flat +/- 1 micron.

[Darin McGrew]

I think Case 4 would be the fastest, but Case 2 would approach Case 4 pretty quickly, as the softer material was polished by the harder material.

[Stan Pope]

How will the sliding Cf's compare?

According to this page, the coefficient of friction between two sliding materials may be governed by, among other things, the "surface roughness of the relative[ly] harder contact surface." If true, then Case 2 results in the lowest coefficient of friction.

That sounds good to me. What, if any, role does the surface roughness of the softer material play in the ultimate Cf?

If the answer is "not much", does this guide us in any way regarding deciding where tospend our time on our PWD wheels and axles?

[FatSebastian]

What, if any, role does the surface roughness of the softer material play in the ultimate Cf?

I believe you have a copy of Jobe's Big Green Book? Does Chapter 9 on Friction and Lubrication give any hints as it relates to plastic deformation?

[*5 J's*]

Identify assumed surface textures, e.g. ripple, sawtooth, etc)

I don't believe there is value to identifying surface textures as the cf is a funtion of surface texture, unless you want to consider the corrugated roofing scenario.

[*5 J's*]

I understand this may be a bit off your topic Stan, but I do think it needs to be considered as I believe when we discuss friction with regards to Pinewood Derby we are missing something. It is always stated that surface area doesn't affect the friction force - and we quote laws of physics to back this up. Obviously, these little cars are not breaking any laws, but I think it has been proven through application that the less surface area, be it tread width or axle surface area, the faster the car. So what's up?

Think about this - take two identical basketballs, one full of air and the other slightly deflated. Roll them along the ground with the same initial velocities. Which will roll furthest? The one that is more full will. What is the difference between the two balls? They both have the same weight. The only difference is that one has more contact surface area than the other.

Check out this site that describes rolling friction. Is this at work in our Pinewood Cars?

I know this site talks about the the friction manifesting as heat - but sound is also a product of friction. All things equal - a wheel that is riding in a knife's edge (V-wheels) will be quieter and faster then a full tread wheel.

Again Stan, I think you are mining for something else in this post and my apologies if I have taken it too far off topic, but I think we need to really consider rolling friction.

[Stan Pope]

The only difference is that one has more contact surface area than the other.

That is a bit of overstatement, I think. There is also flex in the ball's material, for instance.

Rolling friction needs to be separated (to the extent possible) from the discussion of sliding friction. I think that there are an array of different forces at work.

None-the-less, the real world tends to inject variables into our experiments that science tries to isolate out of the discussion. And PWD is a great way to learn about the pure, isoalated relationships and about the more complicated relationships of the real world. Experimental exceptions to "what we think science says" are our keys to greater learning! And as always, clarity in describing the experimental environment that produced such exceptions is important.

[*5 J's*]

The only difference is that one has more contact surface area than the other.

That is a bit of overstatement, I think. There is also flex in the ball's material, for instance.

Rolling friction needs to be separated (to the extent possible) from the discussion of sliding friction. I think that there are an array of different forces at work.

Is friction a function of flex?

[Scubersteve]

well, with flex there is friction hence heat. That's why an underinflated tire runs hotter than a properly inflated one.

[FatSebastian]

The only difference is that one has more contact surface area than the other.

That is a bit of overstatement, I think.

I think 5J's was quoting this board post, which later clarifies "It isn't necessarily the surface area but what comes along with the increased contact surface area. The resistant force to rolling is given a larger moment arm when the ball is flattest." However, the degree to which this happens in a PWD car is not obvious to me at this time. To date I have been satisfied by the hypothesis that reduced wheel-to-track contact (by canting or H-, V-, or U-shaped tread) minimizes interfering contact with random imperfections and microscopic debris on the track surface.

[*5 J's*]

That is a bit of overstatement, I think.

I think 5J's was quoting this board post, which later clarifies "It isn't necessarily the surface area but what comes along with the increased contact surface area. The resistant force to rolling is given a larger moment arm when the ball is flattest." However, the degree to which this happens in a PWD car is not obvious to me at this time. To date I have been satisfied by the hypothesis that reduced wheel-to-track contact (by canting or H-, V-, or U-shaped tread) minimizes interfering contact with random imperfections and microscopic debris on the track surface.

Correct. I'm sure compression is not happening to the extent that a basketball would - but to an extent it is happening. My point of the post is that surface area DOES matter when it comes to losses due to friction despite all the posts to the contrary. With regards to random imperfections and microscopic debris on the track surface, yes I am sure this is part of it, but this may or may not explain why a grooved axle is faster then an no grooved.

[FatSebastian]

this may or may not explain why a grooved axle is faster then an no grooved.

Yes, my comment was not intended to be relevant to axle-to-bore contact, which is a type of journal bearing and therefore a form of sliding friction rather than rolling friction.