WeightAcceleration of GravityForce of GravityGravity at RestNormal ForceNormal Force and WeightAccelerated WeightWeightlessnessWeightWeightAcceleration of GravityAcceleration of GravityObjects that fall to the Earth all experience an Objects that fall to the Earth all experience an acceleration.acceleration.The acceleration due to gravity is The acceleration due to gravity is gg = 9.8 m/s = 9.8 m/s22..This acceleration must be due to a force. This acceleration must be due to a force.Force of GravityForce of GravityThe acceleration of a falling The acceleration of a falling mass mass mm is - is -gg..The force on the mass is The force on the mass is found from found from FF = = mama (action). (action).This gravitational force is This gravitational force is FF = - = -mgmg..gaKinematic viewgmFgravDynamic viewGravity 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.Accelerated WeightAccelerated WeightAn elevator is accelerating An elevator is accelerating downward at 2.0 m/sdownward at 2.0 m/s22..The person has a mass of The person has a mass of 70 kg. 70 kg. What weight is on the scale?What weight is on the scale?Add all the forces, but the Add all the forces, but the net force is –net force is –mama = = FFNN – – mgmg..Solve for Solve for FFNN = = m m ((gg – – aa))The scale shows 56 kg.The scale shows 56 kg.WeightlessnessWeightlessnessIf the elevator accelerated If the elevator accelerated downward at downward at gg, the normal , the normal force would become 0.force would become 0.The person would feel The person would feel weightless.weightless.An object in free fall is An object in free fall is weightless, but not weightless, but not massless.massless.nextMicrogravity research at
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