lecture12 - Friction N f k k N f s s Kinetic friction: f k...

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Unformatted text preview: Friction N f k k N f s s Kinetic friction: f k - friction force N - normal force k - coefficient of kinetic friction Static friction:- coefficient of static friction s s k F N f S W For the book not to fall down, f S = W Pushing harder (increasing F ) increases N and therefore f S,MAX increases, but not the actual value of f S that we had, which needs to continue to be exactly W. Example: When you push a book against a wall, the static friction between the wall and the book can prevent it from falling. If you press harder, the friction force will be: A. Larger than before B. The same C. Smaller than before. Example: A 49 kg rock climber is climbing a chimney between two rock slabs. The coefficient of static friction between her shoes and the rock is 1.2; between her back and the rock it is 0.80. She has reduced her push against the rock until her back and her shoes are on the verge of slipping. What is her push against the rock? W N 1 N 2 f s back f s feet x y N 1 f S back f S feet N 2 w : : , , 2 1 mg f f y N N x feet S back S If the climber dos not slip or move up, the value of the sum of frictional forces is fixed mg f f N N N feet S back S , , 2 1 S...
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This note was uploaded on 10/21/2010 for the course PHYS 221 taught by Professor Herrera-siklody during the Fall '08 term at Iowa State.

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lecture12 - Friction N f k k N f s s Kinetic friction: f k...

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