Net Forces, Friction, Air ResistanceRecall the Sliding Book ExampleForces are Vectors so Directions are ImportantNewton’s 3rd LawFriction is a ForceFriction MechanismWhy Doesn’t Gravity Make the Box Fall?What’s missing in this picture?Wait: We cheated two slides back…Don’t all forces then cancel?Some QuestionsStatic and Sliding (Dynamic) FrictionBig Consequences!“Normal” Forces and Frictional ForcesCoefficient of FrictionStay on the road!Air ResistanceDrag Force Quantified“Free” FallTrajectories with Air ResistanceAir Trajectories, cont.Summary and AssignmentsUCSDPhysics 10Net Forces, Friction, Air ResistanceNet Forces, Friction, Air ResistanceLet’s get realLet’s 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, what’s the force responsible?If the sliding book slows down, what’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 10Newton’s 3Newton’s 3rdrd Law LawFor every action (force), there is an equal and For every action (force), there is an equal and opposite 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)Spring 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 Doesn’t Gravity Make the Box Fall?Why Doesn’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 10What’s missing in this picture?What’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.Spring 2008 9UCSDPhysics 10Wait: We cheated two slides back…Wait: We cheated two slides back…•When we drew the box and floor, with the “normal” force When we drew the box and floor, with the “normal” force from the floor canceling the force of gravity, these weren’t from the floor canceling the force of gravity, these weren’t strictly force pairsstrictly force pairs–but these are the two canceling forces on the box that result in zero acceleration 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 10Don’t all forces then cancel?Don’t all forces then cancel?•How does anything ever move (accelerate) if every force How does anything ever move (accelerate) if every force has 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 of interestinterestForce on boxby floorForce on box by personNet Forceon boxSpring 2008 11UCSDPhysics 10Some QuestionsSome Questions•Does friction always exert a force that tends to Does friction always exert a force that tends to bring things to a halt?bring things to a halt?•What does this say about the direction of the What does this say about the direction of the frictional force, relative to the velocity vector?frictional force, relative to the velocity vector?•What do you think would happen if we loaded What do you think would happen if we loaded lead bricks into the box? Would it become harder lead bricks into the box? Would it become harder to 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 surfacesSpring 2008 13UCSDPhysics 10Big Consequences!Big Consequences!(USGS)Spring 2008 14UCSDPhysics 10““Normal” Forces and Frictional ForcesNormal” Forces and Frictional ForcesWeight of blockDecompose VectorNormalForceFrictionForceWeight of blockReaction ForceFrom Ramp“Normal” meansperpendicularFriction Force = Normal Force  (coefficient of friction)Ffriction = FnormalSpring 2008 15UCSDPhysics 10Coefficient of FrictionCoefficient of FrictionMaterial on MaterialMaterial on Materialss = static friction = static frictionkk = kinetic friction = kinetic frictionsteel / steelsteel / steel0.60.60.40.4add grease to steeladd grease to steel0.10.10.050.05metal / icemetal / ice0.0220.0220.020.02brake lining / ironbrake lining / iron0.40.40.30.3tire / dry pavementtire / dry pavement0.90.90.80.8tire / wet pavementtire / wet pavement0.80.80.70.7Spring 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 s = 0.8 as average for tires on road, Ffriction = 0.8mg•(Normal force is just mg on level surface)Fcurve = macurve = mv2/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.Spring 2008 17UCSDPhysics 10Air ResistanceAir Resistance•We’re always “neglecting air