1. 4.9 Static and Kinetic Frictional Forces There are situations where friction helps us and at other times it is a hindrance. Advantages of friction: Disadvantages of friction: wear and tear & energy loss.

2. Microscopic view of Frictional Forces

3. Frictional force VERSUS Applied force

5. Static Frictional Force The magnitude f s of the static frictional force can have any value from zero up to a maximum value of f s MAX, depending on the applied force. f s ≤ f s MAX f s MAX =  s F N

6. Kinetic Frictional Force The magnitude f k of the kinetic frictional force is given by,

7. Coefficients of Friction

8. EXAMPLE 10 Sled Riding A sled is traveling at 4.00 m/s along a horizontal stretch of snow, as Figure 4.23a illustrates. The coefficient of kinetic friction is  k = 0.0500. How far does the sled go before stopping?

9. 4.10 The Tension Force 56. Part a of the drawing shows a bucket of water suspended from the pulley of a well; the tension in the rope is 92.0 N. Part b shows the same bucket of water being pulled up from the well at a constant velocity. What is the tension in the rope in part b?

10. Object on an Inclined Plane 43. A Mercedes-Benz 300SL is parked on a road that rises above the horizontal. What are the magnitudes of (a) the normal force and (b) the static frictional force that the ground exerts on the tires? 57. A worker stands still on a roof sloped at an angle of above the horizontal. He is prevented from slipping by a static frictional force of 390 N. Find the mass of the worker.