Ward's World + McGraw Hill's AccessScience

38208_Ward's World+McGraw Friction_TeacherKey

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Friction: Test Your Understanding Teacher's Guide + ward ' s science What effect does friction have on mechanisms? How is friction reduced? Friction increases temperatures and wears away at mechanisms. This causes mechanisms to lose functionality over time. Lubricants reduce friction and increase the life of mechanisms. Describe how the explanation of the cause of friction has changed over time. Until about 1940, scientists considered surface roughness to be the main cause of friction, but modern work has discounted the importance of surface roughness. Scientists then believed that the microscopic irregularities of the surfaces touch and push into one another, and the area of these contacting regions is directly proportional to the friction force. This, however, was disproved in the 1970s. In the 1990s, scientists established that friction did not correlate directly with the strength of the adhesive bond itself. Instead, friction is associated with adhesive "irreversibility," or how surfaces behave differently when stuck together compared to being unstuck. Based on the laws of friction, what two factors do not affect the coefficient of friction? What two factors are mainly responsible for the coefficient of friction? The coefficient of friction is not affected by the amount of surface contact and roughness of the surfaces. The friction coefficient is primarily a property of the contacting materials and the contaminants or lubricants at the interface. 1

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