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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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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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