# Why are some surfaces slippery?

Why are ice and oil slippery? In general, why do certain substances make a surface difficult to walk on?

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Water(solid form): Also called ice. –  ACuriousMind Jul 5 at 22:32
Yes yes funny lol. Change it if you would like –  Sciiiiience Jul 5 at 22:34
possible duplicate of Ice skating, how does it really work? –  Draksis Jul 5 at 22:45
@Draksis I don't see how it could be a duplicate, he is referring to surfaces and substances in general and not just water-ice which has its peculiarities and should not be taken as reference for extrapolating to other materials. –  harogaston Jul 5 at 23:32
@harogaston I agree. It isn't a complete duplicate, so I thought I'd leave it up to the OP. –  Draksis Jul 5 at 23:52

## 3 Answers

The simple answer is that the chemical properties of oil and water result in them having smaller coefficients of friction than other surfaces like concrete. The frictional force exerted on a skidding body is given by $F=\mu_k N$ where $N=mg$ is the normal force of the body, and $\mu_k$ is the coefficient of kinetic friction. Assuming the same object skids on both concrete and water, $N$ will be constant while $\mu_k$ will be smaller for water, thus resulting in a smaller frictional force (allowing the object to skid further on water).

Now you may be wondering why $\mu_k$ is smaller for water than concrete. Kinetic friction is primarily caused by chemical bonding between a surface and the object skidding on that surface. In classical mechanics however, it's not necessary to understand the chemical properties of the surfaces being studied. It's just accepted that some surfaces bond stronger than others, with $\mu_k$ being measured for different materials by experiment.

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Great answer, great username. Thanks! –  Sciiiiience Jul 6 at 3:33
This explains why water and oil are slippery but not why ice is slippery, which is still a subject of debate. There's a semi-decent NY Times article about it as well, although it is from 2006 I don't think we've made any breakthroughs on the matter since. Of course neither wikipedia nor the NY Times are the best sources for physics info but there's plenty of interesting citations to go from from there. –  Jason C Jul 6 at 4:02

One word really: friction.

Surfaces that are "slippery" or difficult to walk on have much smaller coefficients of static and kinetic friction. Static friction is why its hard to get things moving and kinetic friction is what slows things down once they start moving.

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Unlike other substances, water is denser in its liquid form. I'd guess that the pressure that you exert when you walk on the ice melts some of it into water, making for a slippery surface

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The pressure-melting theory is falling out of popularity; the reasons are still up for debate; at least as of 2006‌​. –  Jason C Jul 6 at 4:03
Interesting. Thanks for letting me know. –  kingofharts Jul 13 at 23:12