1. © 2005 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials. Friction: Always Rubbing Me the Wrong Way Physics: Principles with Applications, 6 th edition Giancoli

2. Friction On a microscopic scale, most surfaces are rough. The exact details are not yet known, but the force can be modeled in a simple way. Friction is a force that always opposes motion. There are many different types of friction, the two we are concerned with are:  Kinetic or sliding friction – Friction between two surfaces that are moving relative to each other.  Static Friction – Friction between two surfaces that are at rest relative to each other.

3. We can model kinetic or sliding friction with an equation which relates kinetic friction to the normal force: Where is the coefficient of kinetic friction, and is different for every pair of surfaces. Another way to think about kinetic friction is that it is the force that must be overcome to keep an object sliding with a constant velocity.

4. We can model static friction with an inequality: Where is the coefficient of static friction, and is different for every pair of surfaces and is generally larger than the coefficient of kinetic friction. Static friction can be thought of as the force that must be overcome to start an object sliding. It has a maximum value (hence the inequality) and once this value is reached the object will begin to slide.

6. The static frictional force increases as the applied force increases, until it reaches its maximum. Then the object starts to move, and the kinetic frictional force takes over.