1. Friction Friction - force between 2 surfaces that oppose motion Two kinds of friction 1) Static Friction - force of friction that must be overcome to move an object at rest. 2) Kinetic Friction - force of friction that opposes a moving object. **Kinetic friction is weaker than static friction (fk<fs)**

2. Friction Friction Equations F f = uF N **coefficients found in ref. table**

3. Friction Examples 1. A 5kg Steel block is resting on a *horizontal table*. The coefficient of static friction (us) is 0.75 and the Uk is 0.57. a) What minimum force is needed to start this block moving (fs)? b) What is the frictional force on this object as it moves? c) What force must be applied to the object to keep it moving at constant velocity? (Hint: Fnet = 0) 2. A 1000 N car skids on a wet concrete road. If the road is horizontal, what is the friction force on the car? 3. A 53 kg piece of copper is at rest on a horizontal steel table. A force is applied to the box and it achieves an acceleration of 0.1 m/s2. What is the weight of the piece of copper?