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# NIU PHYS 210 - Equilibrium

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EquilibriumForceFundamental ForcesContact ForcesNewton’s LawsFirst Law: Law of InertiaZero Net ForceStatic ForcesVector ForcesForce ComponentsConstant VelocityEquilibriumEquilibriumForceForceForce isForce isA push or pull on an object.A push or pull on an object.A vector with magnitude and A vector with magnitude and direction.direction.Force is notForce is notEnergy.Energy.Power.Power.Momentum.Momentum.Velocity.Velocity.Fundamental ForcesFundamental ForcesGravity is a fundamental force.Gravity is a fundamental force.It acts upon objects from a distance away from the It acts upon objects from a distance away from the source (such as the Earth).source (such as the Earth).There are two other fundamental forces.There are two other fundamental forces.Electroweak force is Electroweak force is common in everyday life.common in everyday life.•ElectricityElectricity•MagnetismMagnetism•LightLight•Radioactive decayRadioactive decayNuclear force is uncommon Nuclear force is uncommon in everyday life.in everyday life.•Nuclear fission (nuclear Nuclear fission (nuclear power plants)power plants)•Nuclear fusion (stars)Nuclear fusion (stars)Contact ForcesContact ForcesMany forces are due to contact between objects.Many forces are due to contact between objects.•Kick a ballKick a ball•Push with a bulldozerPush with a bulldozer•Tug from a ropeTug from a rope•Friction due to the groundFriction due to the groundThe actual force is electricity, but the atoms are so The actual force is electricity, but the atoms are so small we can treat the forces as coming from contact small we can treat the forces as coming from contact by larger objects.by larger objects.Newton’s Laws Newton’s Laws Ancient scientists looked to the Ancient scientists looked to the natural properties of objects.natural properties of objects.•Motion was a result of the object’s Motion was a result of the object’s properties.properties.Newton defined motion based on Newton defined motion based on forces acting from outside an object.forces acting from outside an object.•Motion was the result of external Motion was the result of external forces.forces.Three laws were used to define the Three laws were used to define the behavior of forces on objects.behavior of forces on objects.First Law: Law of InertiaFirst Law: Law of Inertia1An object continues at rest, or in uniform motion in a An object continues at rest, or in uniform motion in a straight line, unless a force is imposed on it.straight line, unless a force is imposed on it.This describes constant velocity, including zero.This describes constant velocity, including zero.No change means no force, and vice versa.No change means no force, and vice versa.rocketno forceconstant velocityZero Net ForceZero Net ForceAn object at rest with no net force is in An object at rest with no net force is in static static equilibriumequilibrium..The net force is due to the sum of forces acting on The net force is due to the sum of forces acting on the object.the object.•The forces are vectorsThe forces are vectors0FFnetStatic ForcesStatic ForcesAn advertising sign weighs An advertising sign weighs 210 N. It is supported from a 210 N. It is supported from a post with a horizontal beam, post with a horizontal beam, and by a chain making an and by a chain making an angle of 35angle of 35 from the from the horizontal. What is the force horizontal. What is the force in the chain?in the chain?W = 210 N = 35ºNewton LegalVector ForcesVector ForcesWith no motion, forces must With no motion, forces must sum to zero.sum to zero.Identify forces on the sign.Identify forces on the sign.•CC is the force on the chain is the force on the chain•BB is the force on the beam is the force on the beam•WW is the weight is the weightVector sum is zero.Vector sum is zero.WCBWBCForce ComponentsForce ComponentsTo find the values, use To find the values, use componentscomponentsFind the vertical components Find the vertical components for the force on the chain.for the force on the chain.•CCyy = C = C sin sin•WWyy = -210 N = -210 N•0 =0 = C Cyy + W + Wyy = C= C sin sin + W + Wyy •C = -WC = -Wyy / / sinsin= 370 N= 370 NUse horizontal components Use horizontal components for the force on the beam.for the force on the beam.•0 =0 = B Bxx + C + Cxx = B = Bxx + (-+ (-C C coscos))•BBxx = C = C cos cos = 300 N = 300 NWy = -210 N = 35ºBxCxCyConstant VelocityConstant VelocityConstant velocity means no Constant velocity means no change in motion.change in motion.Dynamic equilibriumDynamic equilibrium applies applies in states of constant, non-in states of constant, non-zero velocity.zero velocity.Zero net force used here:Zero net force used here:•FFNN + + FFgg + + FFy y = 0= 0•FFxx + + FFfr fr = 0=

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