Why are ice and oil slippery? In general, why do certain substances make a surface difficult to walk on?
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3 Answers
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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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2$\begingroup$ The pressure-melting theory is falling out of popularity; the reasons are still up for debate; at least as of 2006. $\endgroup$– Jason CCommented Jul 6, 2014 at 4:03
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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.
