OC PHYS 112 – The Study of Friction Introduction Friction plays an important part in a large number of daily activities. In spite of its importance, friction is still not well understood. There are a number of empirical laws that describe the friction between two surfaces. These laws are as follows: 1. The ratio of the maximum frictional force and the normal force is a constant and equals the coefficient of friction, , and depends only on the nature of the two surfaces. 2. f s ≤ μ s N and f smax = μ s N 3. f k = μ k N 4. The coefficient of friction is independent of the area of contact. 5. The coefficient of kinetic friction k (the object is in motion) is lower than the coefficient of static friction s (the object is not moving relative to the surface it is resting on). Note : For kinetic friction, much of what follows asks you to consider scenarios where a block is moving at constant speed, so that the acceleration is zero. Do not feel constrained to limit yourself to this line of thinking . Having an acceleration term in your equations is not so bad, provided you have a way of determining that quantity. In fact, it opens up many experimental avenues, and you certainly have the necessary tools to measure ‘ a ’ at your disposal, given your work in previous labs this semester. The configuration shown in Figure 1 can be used to determine the coefficient of static and kinetic friction between a few surfaces. Here, the normal force N = Mg, obtained by balancing forces in the vertical direction on the block. Balancing forces horizontally, the result can be expressed as: mg - f = 0. If the block is sliding at a constant speed then the frictional force f = f k = μ k N = μ k Mg and so μ k = m M .
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