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PH-211 Andres La Rosa CHAPTER 6: Friction Force ____________________________________________________________________ View of the contact surface at a microscopic scale Surfaces are rough at microscopic levels Here we describe an experimental fact: Stationary surface Then, it turns out, to keep the mass M moving at constant velocity, we need to apply a force Fk somewhat smaller than Fs, max Interpretation of the experimental fact described above: Stationary surface Static friction forces Then, it turns out, to keep the mass M moving at constant velocity moving at constant, we need to apply a force Fk somewhat lower than Fs, max Example 2N N' If m = 1 Kg then, Exercise N" m = 1 Kg mg N' Quantifying the friction force STATIC CASE KINETIC CASE Exercise μs = 0.15 CH-6, Problem #25 What is the maximum weight of block-A, for which the system will remain stationary? B A No motion along the vertical direction implies, Maximum static friction force No horizontal motion implies Warm up, for problems involving inclined planes 1) 2) Resolving the force mg into components parallel and perpendicular to the ramp 3) 4) 5) Ff Ff Ch-6 Problem 79. Block A in Fig. 6-56 has mass mA block B has mass mB 4.0 kg, and 2.0 kg. The coefficient of kinetic friction between block B and the horizontal plane is Pk plane is frictionless and at angle T 0.50. The inclined 30°. The pulley serves only to change the direction of the cord connecting the blocks. The cord has negligible mass. Find (a) the tension in the cord and (b) the magnitude of the acceleration of the blocks. Problem #79 General rules for dry sliding friction 1. The friction force is proportional to the normal force at the surface: f = μs N f1 f2 N1 N2 This rule is true only if the objects in contact do not deform appreciable. 2. Usually μs > μk v v== 00 0.1 < μ <1 for many pair of materials v0 However: μ depends very much on surface conditions (cleanliness) v 3. For a given object, the friction force is independent of the apparent area in contact with the supporting surface In both cases the same force F is needed to drag the block. Increasing the load increases the normal force N, and increases the actual contact area fs, max α actual contact area Actual contact area α Hence fs, max α N More quantitatively, fs, max = μ N Key aspects in friction: - The actual contact area is proportional to the normal force - When two objects are forced together, the high peaks of their surface will crumble, bringing more and more area into intimate contact. N Metal-metal friction