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S E C T I O N 5 . 8 • Forces of Friction 133 • The values of - k and - s depend on the nature of the surfaces, but - k is generally less than - s . Typical values range from around 0.03 to 1.0. Table 5.2 lists some re- ported values. • The direction of the friction force on an object is parallel to the surface with which the object is in contact and opposite to the actual motion (kinetic • The coefficients of friction are nearly independent of the area of contact between the surfaces. We might expect that placing an object on the side having the most Quick Quiz 5.11 You press your physics textbook flat against a vertical wall with your hand. What is the direction of the friction force exerted by the wall on the Quick Quiz 5.12 A crate is located in the center of a flatbed truck. The truck accelerates to the east, and the crate moves with it, not sliding at all. What is the direc- Quick Quiz 5.13 You place your physics book on a wooden board. You raise one end of the board so that the angle of the incline increases. Eventually, the Quick Quiz 5.14 You are playing with your daughter in the snow. She sits on a sled and asks you to slide her across a flat, horizontal field. You have a choice of 30 ° F 30 ° (a) (b) F Figure 5.17 (Quick Quiz 5.14) A father pushes his daughter on a sled either by (a) pushing down on her shoulders, or (b) pulling up on a rope. ▲ PITFALL PREVENTION 5.11 The Direction of the Friction Force Sometimes, an incorrect state- |