13lecture2a08

13lecture2a08 - Physics 2A Lecture 13: Feb 6 Vivek Sharma...

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Physics 2A Lecture 13: Feb 6 Vivek Sharma UCSD Physics
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Frictional Forces First Quantitative study by Leonardo da Vinci (1452-1519). ~200 years before Newton’s studies of dynamics area in contact has no effect on friction if load on an object is doubled, its friction is also doubled 200 years before Force was even defined by Newton da Vinci observed: Scientists are still studying the initial fleeting instants of sliding to understand e.g. nanomachines & seismic signals in earthquake
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Quantitative Properties of Friction Static friction applies when object is not moving along surface; kinetic applies when object is moving Expt Both types of friction depend on the normal force on the object n & n ⊥∝ ff G G G G always acts to oppose relative motion of surfaces f G Two kind of friction: Static ( ) and kinetic ( ) sk GG
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Properties Of Friction Expt When a force F is applied to a body, the resulting frictional force has three properties: G || 1. If the body does not move, & component of F || to surface balance each other: F s s f f =− G G G G s n n s coefficient o 2. has a ma f Static frict ximum value , F = normal force on surf io F n ace μ= ss , m a x s,max f ff G || If F exceeds then body starts sliding along su e rfac s,max f G k k k k n 3. When body starts sliding, friction drops to val coefficient of Kine ue tic fr F iction f f n F is a measure of how firmly body presses against surface
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Friction Pictorially
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Friction In Horizontal Motion sk When horizontal force F=230N is applied to a box,it begins to move. Once moving, it takes F=200N to keep it moving at const. velocity. What are & ? μ μ xs , m a x y s,max s Just before motion FT f 0 Fn w 0 f 0.46 n = −= =− = ⇒μ = = xk y k k During motion f 0 w 0 f 0.40 n = = =
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Getting An Angle On Kinetic Friction o Is it harder or easier to keep box moving by pulling it at an angle 30 above horizontal ? What is T now? θ= xk k y System in equilibrium since box moving with v = const. F Tcos f 0 n FT s i n n w Ti 0 n nw s = ⇒θ = μ + = = θ 0 k k : Start by substituting for n in top eqn. w T= ;for 30 T=188N <200N co Solve fo ss i n r T and n μ = θ+μ θ angled is better since n is smaller !
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Sledging In The Winter Wonderland k k Sleigh weighing w accelerates down a snow slope (angle w.r.t horizontal).
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This note was uploaded on 05/08/2008 for the course PHYS 2A taught by Professor Hicks during the Winter '07 term at UCSD.

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13lecture2a08 - Physics 2A Lecture 13: Feb 6 Vivek Sharma...

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