Contact ForceGravity at RestNormal ForceNormal Force and WeightForces on a SlopeNet ForceMechanical AdvantageTension ForceTension or Normal ForceEquilibrium in One DimensionPulleySlide 12Contact ForceContact ForceGravity at RestGravity at RestThe force of gravity acts on The force of gravity acts on all objects all the time.all objects all the time.If an object is at rest, the law If an object is at rest, the law of inertia says that the net of inertia says that the net force is zero.force is zero.There must be a force There must be a force opposite to gravity that opposite to gravity that cancels it out.cancels it out.Fgrav = mgF = Fgrav = mgNormal ForceNormal ForceThe force that opposes The force that opposes gravity for objects on the gravity for objects on the ground is called the ground is called the normal normal forceforce..It is perpendicular (normal) It is perpendicular (normal) to the plane of the ground.to the plane of the ground.The force is a result of the The force is a result of the law of reaction.law of reaction.FN = mgFsb = Fbs = mgNormal Force and WeightNormal Force and WeightThe normal force pushing up The normal force pushing up against gravity can be measured.against gravity can be measured.We measure weight with a scale We measure weight with a scale that measures normal force.that measures normal force.Weight is a force, not a mass.Weight is a force, not a mass.Pounds measure weight, so force Pounds measure weight, so force (not mass) can be measured in (not mass) can be measured in pounds. pounds.Forces on a SlopeForces on a SlopeNFDraw the forces acting on Draw the forces acting on the block.the block.•The force of gravity points The force of gravity points down with magnitude down with magnitude FFgg==mgmg..•The normal force The normal force FFNN points points away from the surface of the away from the surface of the inclined plane.inclined plane.•An applied force An applied force FFappapp holds holds the block in place.the block in place.mmgFgappFNet ForceNet ForcecosmgFNThe block stays in place on The block stays in place on the plane.the plane.•No net force along the No net force along the surfacesurface•No net force away from the No net force away from the surfacesurfaceFind the components of Find the components of gravity in those directions.gravity in those directions.Find the unknown forcesFind the unknown forcesmmgFgcosmgsinmgsinmgFappThe diagram is the same for constant velocity!Mechanical AdvantageMechanical AdvantageThe inclined plane has been The inclined plane has been used as a machine for used as a machine for centuries.centuries.Pushing a block up an Pushing a block up an incline is easier than lifting.incline is easier than lifting.The ratio of the force to lift The ratio of the force to lift directly compared to the directly compared to the force using the incline is the force using the incline is the mechanical advantagemechanical advantage..mmgsin1sinmgmgsinmgTension ForceTension ForceA taut rope has a force A taut rope has a force exerted on it.exerted on it.If the rope is lightweight and If the rope is lightweight and flexible the force is uniform flexible the force is uniform over the entire length.over the entire length.This force is called This force is called tensiontension and points along the rope.and points along the rope.Tension acts on both ends.Tension acts on both ends.Fg = mgFT = Fg = mgFT-FTforces on the ropeforces on the ballTension or Normal ForceTension or Normal ForceTension and normal forces are different.Tension and normal forces are different.•A pull on an object A pull on an object tension tension•A push from a surface A push from a surface normal force normal forceEither one or both may be present.Either one or both may be present.mNormal forceFNFfrFgTension forceFTEquilibrium in One Equilibrium in One DimensionDimensionTwo weights are hung Two weights are hung supported by strings.supported by strings.•On the lower block the two On the lower block the two forces balance: forces balance: FFTT22 = = mm22gg•On the upper block there On the upper block there are three: are three: FFTT11 = = mm11g + Fg + FTT22 •FFTT11 = ( = (mm11 + m + m22))ggThe upper string has more The upper string has more tension than the lower string.tension than the lower string.m1FT1m1g3 forces on the upper blockFT2m2m2gFT22 forces on the lower blockPulleyPulleyA pulley uses tension to A pulley uses tension to transfer a force to another transfer a force to another direction.direction.m1m2m1m1m2m2FropeFfrFropeFgFTFTforces on block 1 forces on block 2forces on the ropeMechanical AdvantageMechanical AdvantageWith more than one pulley With more than one pulley the force needed to lift an the force needed to lift an object can be reduced.object can be reduced.The pulley is a simple The pulley is a simple machine.machine.The mechanical advantage The mechanical advantage to the left is 2. to the left is 2.
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