Item — HN Reader

So it seems to me that friction is a pretty dicey thing to measure. It seems that friction is really a property of each of the two surfaces. For example, if we are measuring the friction between fingers and a smooth surface, you would notice that fingerprints reduce the friction. But what about the hand gripping a tree branch? Might not the friction be increased in that sort of surface-to-surface contact? Or hanging onto a hunk of the fur of a running buffalo for dear life?

Their approach seems to imply relatively smooth surfaces: "Because there are all the gaps between the fingerprints, what they do is reduce the contact area with the surface"

I am not seeing any reference to the fractal nature of surface-to-surface contact here.

2 comments

Also grip tends to have little to do with friction. When moving heavy objects you curl your fingers around it vs. laying your hand flat and trusting in friction. If anything reducing friction might be a good idea because you can better position your hands for optimum grip.

Edit: When you move a couch you curl you fingers under an edge, hold onto a tree branch you curl your fingers around it etc. Climbing a tree can involve friction but usually it’s griping the texture of the bark.

PS: Consider how you would hold a wet wine glass.

I have to disagree based on my weightlifting experience. When deadlifting heavy weight (1.5x bodyweight and higher), friction makes a big impact on my grip. Sweaty hands, and even a bar with worn-down grooves, can significantly degrade my grip. This is why weightlifters (and gymnasts) use chalk.

Chalk does not maximize friction in all cases. With light pressure chalk reduces friction, under heavy pressure it increases it. Which is clearly a best case situation.

As an example of this pool players use chalk to smoooth out their stroke.

I don't think that's a function of pressure, but amount. If my hands were caked in chalk, I probably would have a harder time maintaining grip. But if I have just enough to maintain dry hands, but not enough to reduce friction, then it helps.

No, damp hands stick to the pool que. But a vary light amount of chalk fixes that problem. Friction is actually a fairly complex activity where you can change both the contact friction and the rate of friction increase under pressure in different directions.

You can slip on a vary thin layer of spilled sand, but trains use it to increase friction between the wheels and the smooth track.

For heavy objects, it's probably not so relevant.

But in my experience, more friction would probably be useful when holding tools/weapons. Hammers have a rubber grip, for example. I've noticed I lose smooth eskrima sticks far more often than ones I've taped. In an natural selection event, that could be lethal [1].

[1] http://www.youtube.com/watch?v=c_qwaXb6kAE

Tire treads?

Tire treads are mostly to prevent hydroplaning.

(Bicycle tires can't hydroplane, at least not at bicycle speed, and therefore road bike tires don't have treads. Treads reduce the contact area between the tire and the road, resulting in worse traction than a perfectly smooth tire.)

Hmmm, so do fingerprints prevent hydroplaning?

I have never noticed my fingers hydroplaning, so apparently yes. Mystery solved!

The earlier example of grasping a wet wine glass. That would seem to be an example.