# Why are some surfaces slippery?

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

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.