2. Friction The force that opposes a sliding motion. Enables us to walk, drive a car, etc. Due to microscopic irregularities in even the smoothest of surfaces.

3. There are two types of friction Static friction exists before sliding occurs Kinetic friction exists after sliding occurs In general f k <= f s

4. Friction and the Normal Force The frictional force which exists between two surfaces is directly proportional to the normal force. That’s why friction on a sloping surface is less than friction on a flat surface.

5. Static Friction f s   s N f s : static frictional force (N)  s : coefficient of static friction N: normal force (N) Static friction increases as the force trying to push an object increases… up to a point!

6. A force diagram illustrating Static Friction Applied ForceFrictional Force Normal Force Gravity

7. A force diagram illustrating Static Friction Bigger Frictional Force Normal Force Gravity Bigger Applied Force

8. A force diagram illustrating Static Friction Frictional Force Normal Force Gravity Even Bigger Applied Force The forces on the book are now UNBALANCED! Static friction cannot get any larger, and can no longer completely oppose the applied force.

9. Kinetic Friction f k =  k N f k : kinetic frictional force (N)  k : coefficient of kinetic friction N: normal force (N) Kinetic friction (sliding friction) is generally less than static friction (motionless friction) for most surfaces.

10. Determination of the Coefficients of Friction Coefficient of Static Friction 1) Set a block of one material on an incline plane made of the other material. 2) Slowly increase angle of plane until the block just begins to move. Record this angle. 3) Calculate  s = tan .

11. Determination of the Coefficients of Friction Coefficient of Kinetic Friction 1) Set a block of one material on an incline plane made of the other material. 2) Slowly increase angle of plane until the block just begins to move at constant speed after giving it a slight tap. Record this angle. 3) Calculate  k = tan .