Where the Rubber Meets the Road Friction

Friction A force that OPPOSES MOTION. What causes friction? Attraction between protons and electrons in the object and surface + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - +

Friction What causes friction? 2. Small ridges and bumps at the microscopic level cause objects and surfaces to catch on one another.

Friction Friction force strength How “sticky” are the surfaces involved? Coefficient of friction (μ) – NO UNITS How hard are the surfaces pushing together? NORMAL FORCE Are the surfaces sliding against each other? KINETIC vs. STATIC

Which Coefficient? Kinetic Static Used ONLY for SLIDING objects Pulled along a surface Skidding Static Used for any NON-SLIDING objects Not moving Rolling

Example #1 A 20 newton wooden box is sliding across a wooden surface. What is the friction force acting on the box? Ff = μFN Ff = (0.3)(20 N) Ff = 6.0 N

Example #2 A 40 newton copper box is placed on a flat, steel table. What is the minimum force needed to make the box move? Ff = μFN Ff = (0.53)(40 N) Ff = 21 N

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