WorkHard Work?Force over a DistanceCreating MotionWork in a DirectionBeginning and EndScalar ProductThe JouleConstant ForceIndividual ForceDissipative ForceWorkWorkHard Work?Hard Work?True or false, did you do work on the object listed?True or false, did you do work on the object listed?•You hold a book in one hand while standing for 10 minutes.You hold a book in one hand while standing for 10 minutes.•You start a resting ball rolling across a level table.You start a resting ball rolling across a level table.•You move a heavy box from one desk top to another of You move a heavy box from one desk top to another of equal height.equal height.•You carry a pen up a flight of stairs.You carry a pen up a flight of stairs.Force over a DistanceForce over a DistanceIn physics, work is the force In physics, work is the force acting on object over a acting on object over a distance.distance.Forces acting on objects at Forces acting on objects at rest do no work. rest do no work.Creating MotionCreating MotionWork is force acting over a Work is force acting over a distance.distance.•Changing motion requires a Changing motion requires a force: F = ma.force: F = ma.•Force is applied over a finite Force is applied over a finite distance.distance.Forces that change motion Forces that change motion maymay do work. do work.Fv = 0v > 0Work in a DirectionWork in a DirectionThe force must act in the direction of the The force must act in the direction of the displacement to be work.displacement to be work.Force acts in this directionThe object moves in this directionNot all the force is in the direction of motion.Beginning and EndBeginning and EndOnly the force acting in the Only the force acting in the direction of displacement counts direction of displacement counts towards work.towards work.•Displacement considers the Displacement considers the beginning and end points.beginning and end points.Forces acting perpendicular to an Forces acting perpendicular to an object’s motion do no work.object’s motion do no work.•Linear motion with perpendicular Linear motion with perpendicular forcesforces•Uniform circular motionUniform circular motionFGFGFNFNScalar ProductScalar ProductFinding the component of a vector in the direction of Finding the component of a vector in the direction of another vector is called the another vector is called the scalar productscalar product..||ABAdirA BA||cos||ABBABABASBAPerpendicular vectors Perpendicular vectors always have a scalar always have a scalar product of zero.product of zero.The JouleThe JouleWork is force acting over a distance, and has units Work is force acting over a distance, and has units equal to units of force times units of distance.equal to units of force times units of distance.•With 1-dimensional constant force, With 1-dimensional constant force, WW = = FF xx•With vectors and a constant force, With vectors and a constant force, WW = = FF xx cos cosThe unit of work is the joule (J) = 1 N m.The unit of work is the joule (J) = 1 N m.Constant ForceConstant ForceLifting an object requires a Lifting an object requires a constant force equal to constant force equal to gravity, gravity, F F = = mgmg..The work done by the lifter is The work done by the lifter is WW = = FF x = x = mghmgh..The work done by gravity is The work done by gravity is WW = - = - FF x = -x = -mghmgh..x = h-mg-mgmgmgIndividual ForceIndividual ForceWork can be done by Work can be done by individual forces, or by a net individual forces, or by a net force.force.•Constant velocity has net Constant velocity has net force of zero.force of zero.•A net force of zero may A net force of zero may include forces that are not include forces that are not zero.zero.Force acting through a Force acting through a distance applies to separate distance applies to separate forces.forces.FxFgFfrFNDissipative ForceDissipative ForceA man is letting a 300 kg A man is letting a 300 kg piano slide 4 m at constant piano slide 4 m at constant velocity down a 30velocity down a 30°° incline incline while exerting a 400 N force while exerting a 400 N force on the horizontal. What work on the horizontal. What work does he do?does he do?•The component of the force The component of the force is (400 N)(cos 30) = is (400 N)(cos 30) = 350 N350 N•Negative since it is opposite Negative since it is opposite the displacementthe displacement•The work is (-350 N)(4 m) = The work is (-350 N)(4 m) = -1400 N m = -1400 J-1400 N m = -1400
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