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Like, for example, rubber is a substance with high friction, as an object with neither high or low friction would easily stop on it, and wouldn't skid. Ice is considered an object with low friction, as an object with neither high or low friction wouldn't easily stop on it, and would skid.

But, what about water?

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I guess that this term is only meaningful for solids. Liquids change their shape and the resistance to do so is measured by viscosity, not friction. Water has a low viscosity relatively to 'thick' substances like honey. But you can't compare liquids with solids -like apples with oranges. Also, whether the friction on a wet surface is high depends on the thickness of the water column, surface tension, and so on... –  Luboš Motl Oct 15 '12 at 6:19
Objects are not high or low friction on their own. You can only talk about friction between surfaces. Rubber usually has a lot of friction with other things, but in contact with a frictionless surface, it is frictionless. So rubber in contact with a low-friction surface would mean there is not much friction, because the rubber has low friction with that surface. –  Jim Mar 30 at 15:10

2 Answers 2

up vote 4 down vote accepted

Friction is caused by two physical processes, both of which dissipate energy. It's the dissipation of energy that means work is required to slide over the surface, and this work is why we feel a frictional force.

Anyhow, the first factor is the surface energy of the material because this affects the adhesion between the sliding object and the substrate. A high surface energy like steel means a high friction and low surface energy like PTFE means a low friction. The second factor is how much energy is dissipated by viscoelastic losses in the bulk of the substrate. This is the main reason rubber has such high friction, because as an object slides over it the rubber deforms and absorbs energy. Rubber actually has quite a low surface energy but the viscoelastic losses dominate the friction.

Anyhow, with water neither of these criteria can be applies because water is a liquid, so you can't assign a friction coefficient to water.

Having said this, a film of water can be a very effective lubricant. For example if you step out of the shower onto a smooth lino floor you're likely to discover the hard way what a good lubricant a film of water can be.

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I would say that the friction coeficient would make sense for liquid as well if the internal cohesion forces in the object are comparable with the friction superficial contact forces. Imagine a drop of mercury on a flat glass surface; it stays almost spherical and it slides very easily - the friction to glass is comparable to the internal cohesion forces. This is the same as having a drop of water on a surface treated with an ultra-hydrophobic substance (e.g. certain nanopolymers) See link

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protected by Qmechanic Mar 30 at 15:08

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