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Unformatted text preview: Lecture 11 Friction. Dynamics of Circular Motion ACT: Scale ACT: Scale A 40N block is hung from a rope attached to a scale. The scale is then attached to a wall and reads 40N. What will the scale read when it is instead attached to another 40N block, as shown in the second figure? A. 0 N B. 40N C. 80N T 1 T 1 mg The scale has a = 0 T 1 T 1 T 1 = T 2 T 2 T 2 String 2 is massless (and has a = 0) T 2 is the same on both sides T 2 The block has a = 0 T 2 = mg mg T 2 mg The forces on the scale are exactly the same! DEMO Friction Friction f k box, table v Friction always opposes relative motion. Solid-solid Solid-fluid Fluid-fluid Surface friction (kinetic or static) Drag forces Viscosity It is caused by microscopic interactions between two surfaces. Surface friction (solid-solid) Kinetic friction: When the relative motion between the two objects is not zero. It slows down the sliding motion. Static friction: When the relative motion between the two objects is zero. It prevents the sliding motion from happening. Example: A box on an incline that doesnt slide down. Example: A box sliding on a horizontal table will eventually stop. Kinetic friction Kinetic friction When the relative motion between the two objects is not zero. It slows down the sliding motion. Experimentally, it is observed that the kinetic friction force between two surfaces: Is parallel to the surface (and thus perpendicular to the normal) Its magnitude does not depend on the speed (except when v = 0) or on the area of the contact surfaces. Its magnitude is proportional to the magnitude of the normal force between the two surfaces: f k = k N k = coefficient of kinetic friction (depends on the materials) normal force between the two surfaces: Use the correct one!!! ( N is NOT always mg ) EXAMPLE: Box on incline with friction EXAMPLE: Box on incline with friction...
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