# d_forces2 - PHYS 81 1st Sem 2009-2010 AKGTapia Frictional...

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PHYS 81 1st Sem 2009-2010 AKGTapia 1 Frictional Force-”A Contact Force” • Force which opposes the motion of a body η μ = f μ is the coefficient of friction (depends on the nature of the contact surfaces) η is the normal force and •f is the frictional force Note: Subscript s and k for f or μ indicate static and kinetic, respectively. Kering - keri ! •Static friction -frictional force present for bodies at rest f smax μ s η •Kinetic friction -frictional force present for bodies moving. f k = μ k η •Ideally, frictional force does not depend on the area of contact but on the normal force and coefficient of friction. •Rolling friction - observed when an object (i.e. wheels) rolls at constant speed along a horizontal road without slipping μ r =(F to keep v=k)/(Normal force) μ s > μ k F app η F W f F app η F W f Indentifying the forces (weight, normal force, applied forces, contact forces, etc…) FBD- F ree- b ody D iagram Force vectors in cartesian coordinates for each body Example : μ s between a car’s tires and the road on a particular day is 0.7. What is the steepest angle of inclination of the road for which the car can be parked with its wheels locked and not slide down the hill? θ η f s F w θ η f s F w = + - = 0 sin θ F f F w s x = - = 0 cos θ F η F w y s μ = s f θ μ tan cos sin = = = mg mg f s s 0 1 35 ) 7 . 0 ( tan = = - Announcements • This week: make sure to pick your question for the project • Next week: prepare a brief proposal for the project • July 31: Deadline of problem set #3 • August 11: Deadline of the 1 st Draft of your essays • August 24: 2 nd Long Exam, PSLH-B, 5-7 pm Example : Given an angle where the box will accelerate downwards, compute the acceleration of the box when friction is present. θ η f k F w θ η f k F w = + - = ma sin θ F f F w k x = - = 0 cos θ F η F w y ( ) cos θ μ sin θ g a k - =

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PHYS 81 1st Sem 2009-2010 AKGTapia 2 Problems: A car is travelling at 30m/s along a horizontal road. The coefficients of friction between the road and the tires are μ s =0.5 and μ k =0.3. How far does the car travel before stopping if (a) the car is braked firmly so that the wheels almost slip and (b) the car is braked so hard so that the wheels lock? (a) Static Friction
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d_forces2 - PHYS 81 1st Sem 2009-2010 AKGTapia Frictional...

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