4/18/2008Lecture 8 1UCSDPhysics 10Net Forces, Friction, Air ResistanceNet Forces, Friction, Air ResistanceLetLet’’s get reals get realSpring 2008 2UCSDPhysics 10Recall the Sliding Book ExampleRecall the Sliding Book Example••Why do things not continue to move at constant velocity?Why do things not continue to move at constant velocity?– Don’t things strive to be at rest (I know I do)?••If the sliding book slows down, whatIf the sliding book slows down, what’’s the force responsible?s the force responsible?– How could I keep it moving at a constant velocity?– Do I need to apply a force to keep it moving? Why?Spring 2008 3UCSDPhysics 10Forces are Vectors so Directions are ImportantForces are Vectors so Directions are ImportantForce #1Force #2Force #1Force #2Total Force Total Force = 0Spring 2008 4UCSDPhysics 10NewtonNewton’’s 3s 3rdrd Law LawFor every action (force), there is an equal andFor every action (force), there is an equal andopposite reaction (opposing force).opposite reaction (opposing force).Force on Newton by Einstein = Force on Einstein by Newton(But their accelerations need not be the same: Newton tries tooutsmart Einstein by loading his own cart with lead bricks)4/18/2008Lecture 8 2Spring 2008 5UCSDPhysics 10Friction is a ForceFriction is a ForceForce on person by boxForce on floor by boxForce on box by floorIt’s the sum of all the forces that determines the acceleration.Every force has an equal & opposite partner.Force on box by personSpring 2008 6UCSDPhysics 10Friction MechanismFriction MechanismCorrugations in the surfaces grind when things slide.Lubricants fill in the gaps and let things slide more easily.Spring 2008 7UCSDPhysics 10Why DoesnWhy Doesn’’t Gravity Make the Box Fall?t Gravity Make the Box Fall?Force of Earth acting on Box (weight)Force of Floor acting on Box Force from floor on boxcancels gravity.If the floor vanished, thebox would begin to fall.Spring 2008 8UCSDPhysics 10WhatWhat’’s missing in this picture?s missing in this picture?Force on person by boxForce on floor by boxForce on box by floorForce on box by personA pair of forces acting between person and floor.4/18/2008Lecture 8 3Spring 2008 9UCSDPhysics 10Wait: We cheated two slides backWait: We cheated two slides back……••When we drew the box and floor, with the When we drew the box and floor, with the ““normalnormal”” force forcefrom the floor canceling the force of gravity, these werenfrom the floor canceling the force of gravity, these weren’’ttstrictly force pairsstrictly force pairs– but these are the two canceling forces on the box that result in zeroacceleration of the box••The real pairs have to involve the earth:The real pairs have to involve the earth:boxfloorearthsatellite• Force Pairs:• earth-box (grav)• box-floor (contact)• earth-satellite (grav)Spring 2008 10UCSDPhysics 10DonDon’’t all forces then cancel?t all forces then cancel?••How does anything ever move (accelerate) if every forceHow does anything ever move (accelerate) if every forcehas an opposing pair?has an opposing pair?••The important thing is the The important thing is the net forcenet force on the object of on the object ofinterestinterestForce on boxby floorForce on box by personNet Forceon boxSpring 2008 11UCSDPhysics 10Some QuestionsSome Questions••Does friction always exert a force that tends toDoes friction always exert a force that tends tobring things to a halt?bring things to a halt?••What does this say about the direction of theWhat does this say about the direction of thefrictional force, relative to the velocity vector?frictional force, relative to the velocity vector?••What do you think would happen if we loadedWhat do you think would happen if we loadedlead bricks into the box? Would it become harderlead bricks into the box? Would it become harderto slide?to slide?••What are some ways to reduce frictional forces?What are some ways to reduce frictional forces?Spring 2008 12UCSDPhysics 10Static and Sliding (Dynamic) FrictionStatic and Sliding (Dynamic) Friction••Static frictional force: when nothing is slidingStatic frictional force: when nothing is sliding••Sliding frictional force: when surfaces are slidingSliding frictional force: when surfaces are sliding••Static frictional forces always greater than sliding onesStatic frictional forces always greater than sliding ones• Lubrication provides microscopic rollers between surfaces4/18/2008Lecture 8 4Spring 2008 13UCSDPhysics 10Big Consequences!Big Consequences!(USGS)Spring 2008 14UCSDPhysics 10““NormalNormal”” Forces and Frictional Forces Forces and Frictional ForcesWeight of blockDecompose VectorNormalForceFrictionForceWeight of blockReaction ForceFrom Ramp“Normal” meansperpendicularFriction Force = Normal Force (coefficient of friction)Ffriction = µFnormalSpring 2008 15UCSDPhysics 10Coefficient of FrictionCoefficient of Friction0.70.70.80.8tire / wet pavementtire / wet pavement0.80.80.90.9tire / dry pavementtire / dry pavement0.30.30.40.4brake lining / ironbrake lining / iron0.020.020.0220.022metal / icemetal / ice0.050.050.10.1add grease to steeladd grease to steel0.40.40.60.6steel / steelsteel / steelµµkk = kinetic friction = kinetic frictionµµss = static friction = static frictionMaterial on MaterialMaterial on MaterialSpring 2008 16UCSDPhysics 10Stay on the road!Stay on the road!••What does it take to stay on the road around a curve?What does it take to stay on the road around a curve?– using µs = 0.8 as average for tires on road, Ffriction = 0.8mg• (Normal force is just mg on level surface)Fcurve = macurve = mv2/r•where r is radius of curve, say 50 m (e.g., cloverleaf exit ramp)••Got enough friction if Got enough friction if FFcurvecurve < < FFfrictionfriction– happens if v2 < 0.8gr, or v < 20 m/s = 44 m.p.h.4/18/2008Lecture 8 5Spring 2008 17UCSDPhysics 10Air ResistanceAir Resistance••WeWe’’re always re always ““neglecting air resistanceneglecting air resistance”” in physics in physics– Can be difficult to deal with••Affects projectile motionAffects projectile motion– Friction force opposes velocity through medium– Imposes horizontal force, additional vertical forces– Terminal velocity for falling objects••Dominant energy drain on cars, bicyclists, planesDominant energy drain on
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