Static FrictionHolding in PlaceInequalityStarting MotionMeasuring Friction: PulleyCoefficient of Friction: PulleyMeasuring Friction: InclineCoefficient of Friction: InclineNormal Force and FrictionStatic FrictionStatic FrictionHolding in PlaceHolding in PlaceObjects on an incline will often stay put.Objects on an incline will often stay put.There must be a force that holds the object in place.There must be a force that holds the object in place.Static frictionStatic friction is from the contact of resting objects. is from the contact of resting objects.•Force holds up to a certain pointForce holds up to a certain point•Force is based on the type of contact (rough, smooth)Force is based on the type of contact (rough, smooth)•Maximum force is proportional to the pressing force of the Maximum force is proportional to the pressing force of the object (normal force)object (normal force)InequalityInequalityThe approximate formula for static friction is:The approximate formula for static friction is:ss is the coefficient of static friction is the coefficient of static frictionThis is an This is an inequalityinequality..•The force of static friction is generally less than the The force of static friction is generally less than the coefficient times the normal forcecoefficient times the normal forceNsfrFFStarting MotionStarting MotionIf If FFfrfr < < ssFFNN = = ssmgmg cos cos then the block will hold.then the block will hold.At equality the block just At equality the block just begins to move.begins to move.NFmmgFgcosmgsinmgsinmgFfrtancossinssNsfrmgmgFFMeasuring Friction: PulleyMeasuring Friction: Pulleym1m2FropegmFsfr 1(max)FTFTFropegmFg 2Use a pulley and vary the Use a pulley and vary the weight to see when weight to see when movement begins.movement begins.•Forces balance on mass 1Forces balance on mass 1•Tension is equal on the ropeTension is equal on the rope•Forces balance on mass 2Forces balance on mass 2Coefficient of Friction: Coefficient of Friction: PulleyPulleym1m2gmFsfr 1(max)gmFg 21221/ mmgmgmssThe pulley determines the The pulley determines the coefficient of friction through coefficient of friction through the ratio of masses.the ratio of masses.Measuring Friction: InclineMeasuring Friction: InclineNFmmgFgcosmgsinmgcos(max)mgFsfrUse an incline and vary the Use an incline and vary the angle to see when angle to see when movement begins. movement begins. •One component of gravity One component of gravity balances the normal forcebalances the normal force•The other component of The other component of gravity balances frictiongravity balances frictionCoefficient of Friction: Coefficient of Friction: InclineInclinemsinmgcos(max)mgFsfrtancossinssmgmgThe incline determines the The incline determines the coefficient of friction through coefficient of friction through the tangent of the angle. the tangent of the angle.Normal Force and FrictionNormal Force and FrictionStatic friction depends on both the normal force and Static friction depends on both the normal force and on the coefficient of friction.on the coefficient of friction.To reduce friction requires reducing one of those To reduce friction requires reducing one of those factors.factors.•Reduce normal force by lightening the loadReduce normal force by lightening the load•Reduce normal force by adding additional upward forceReduce normal force by adding additional upward force•Add a lubricant to reduce the coefficient of frictionAdd a lubricant to reduce the coefficient of
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