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# Friction

## The resistance that one surface or object encounters when moving over another.

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Slippery When Wet

### Slippery When Wet

Whether it’s a freshly moped floor, or an icy sidewalk, it is common knowledge that wet surfaces can be slippery. The slippery behavior is due to the decrease in the frictional coefficient μ\begin{align*}\mu\end{align*}.

#### Explore More!

• Friction is a force that opposes motion. Without it, you wouldn’t be able to walk, drive a car or even get out of bed. The frictional force has two forms, static and kinetic. Static friction is the friction of an object that is stationary, and kinetic friction is the friction of motion. Between the two frictional forces static friction is larger than kinetic friction.
• Both forces are equal to μs,kN\begin{align*}\mu_{s,k}N\end{align*}, where s\begin{align*}s\end{align*} is used to represent the static case or k\begin{align*}k\end{align*} is used to represent the kinetic case and N\begin{align*}N\end{align*} represents the normal force on the object. To understand the importance of friction in your daily life, image the forces that are present between your foot and the ground as you walk. With each step you are pulling your leg backward. Because the acceleration vector is pointing backward at the interface between your foot and the ground, the frictional force must be pointing forward. So without friction, you wouldn’t even be able to walk!

#### Show What You Have Learned?

Using the information provided above, answer the following questions.

1. When someone is driving a car down the street what direction is the frictional force point at the interface between the tire and the ground?
2. If you were able to decrease the frictional coefficient (either static or kinetic) would the frictional force increase or decrease?
3. Even though it wasn’t explained, what hypothesis can you formulate to explain why the coefficient of static friction is greater than the coefficient of kinetic friction?

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