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Work, Power & Energy Work, Power & Energy Chapter 4 Explaining the Causes of Motion in a Different Way

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Work lThe product of force and the amount of displacement along the line of action of that force. Units: ft. lbs (horsepower) Newtonmeter (Joule) e

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Work = F x d To calculate work done on an object, we need: The Force ¬The average magnitude of the force The direction of the force The Displacement ¬The magnitude of the change of position The direction of the change of position

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Calculate Work lDuring the ascent phase of a rep of the bench press, the lifter exerts an average vertical force of 1000 N against a barbell while the barbell moves 0.8 m upward lHow much work did the lifter do to the barbell?

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Calculate Work Table of Variables: Force = N Displacement = +0.8 m Force is positive due to pushing upward Displacement is positive due to moving upward

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Calculate Work Table of Variables: Force = N Displacement = +0.8 m Select the equation and solve:

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Work performed climbing stairs lWork = Fd lForce l Subject weight lFrom mass, ie 65 kg lDisplacement l Height of each step lTypical 8 inches (20cm) lWork per step l 650N x 0.2 m = Nm lMultiply by the number of steps

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Work on a stair stepper lWork = Fd lForce l Push on the step l???? lDisplacement l Step Height l8 inches lWork per step l ???N x.203 m = ???Nm

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Energy lEnergy (E) is defined as the capacity to do work (scalar)Energy l Many forms lNo more created, only converted lchemical, sound, heat, nuclear, mechanical lMechanical Energy l Kinetic Energy (KE): l energy due to motion l Potential Energy (PE): lenergy due to position

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Kinetic Energy Energy due to motion reflects l the mass l the velocity of the object KE = 1/2 mv 2

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Calculate Kinetic Energy How much KE in a 5 ounce baseball (145 g) thrown at 80 miles/hr (35.8 m/s)?

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Calculate Kinetic Energy Table of Variables Mass = 145 g kg Velocity = 35.8 m/s

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Calculate Kinetic Energy Table of Variables Mass = 145 g kg Velocity = 35.8 m/s Select the equation and solve: KE = ½ m v 2 KE = ½ (0.145 kg)(35.8 m/s) 2 KE = ½ (0.145 kg)( m/s/s) KE = ½ (185.8 kg m/s/s) KE = 92.9 kg m/s/s, or 92.9 Nm, or 92.9J

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Gravitational PE lAffected by the objects l weight lmg l elevation (height) above reference point l ground or some other surface lh GPE = mgh Units = Nm or J (why?)

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Calculate GPE How much gravitational potential energy in a 45 kg gymnast when she is 4m above the mat of the trampoline?

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Calculate GPE GPE relative to mat Table of Variables m = 45 kg g = m/s/s h = 4 m PE = mgh PE = 45kg * m/s/s * 4 m PE = J

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Conversion of KE to GPE and GPE to KE and KE to GPE and …

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Work - Energy Relationship lWork is the change in the mechanical energy of the object

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Work - Energy Relationship lIf more work is done, greater energy l greater average force l greater displacement

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Extension…

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Power lThe rate of doing work l Work = Fd Units: Fd/s = J/s = watt

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Calculate & compare power lDuring the ascent phase of a rep of the bench press, two lifters each exert an average vertical force of 1000 N against a barbell while the barbell moves 0.8 m upward lLifter A: 0.50 seconds lLifter B: 0.75 seconds

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Calculate & compare power Lifter A Table of Variables F = 1000 N d = 0.8 m t = 0.50 s Lifter B

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Power on a cycle ergometer lWork = Fd lForce: 3kg lDisplacement: 6m /rev lWork per revolution l 3kg x 6 m = 18 kgm l60 rev/min

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Power on a cycle ergometer lWork = Fd lForce: 3kg lDisplacement: 6m /rev lWork per revolution l 3kg x 6 m = 18 kgm l60 rev/min 1 Watt = 6.12 kgm/min

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Compare power in typical stair stepping lWork = Fd lForce: Push on the step lconstant setting lDisplacement l Step Height: 5 vs 10 l0.127 m vs m lstep rate l 56.9 /min vs 28.8 /min lTime per step l 60s/step rate Thesis data from Nikki Gegel and Michelle Molnar

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Compare power in typical stair stepping lWork = Fd lForce: Push on the step lconstant setting lDisplacement l Step Height: 5 vs 10 l0.127 m vs m lstep rate l 56.9 /min vs 28.8 /min

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Compare power in typical stair stepping lWork = Fd lForce: Push on the step lconstant setting lDisplacement l Step Height: 5 vs 10 l0.127 m vs m lstep rate l 56.9 /min vs 28.8 /min Results: VO 2 similar fast/short steps vs slow/deep steps

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- & + Work lPositive work is performed when the direction of the force and the direction of motion are the same l ascent phase of the bench press l Throwing a ball l push off (upward) phase of a jump

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- & + Work lPositive work lNegative work is performed when the direction of the force and the direction of motion are the opposite l descent phase of the bench press l catching l landing phase of a jump

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Calculate Work lDuring the descent phase of a rep of the bench press, the lifter exerts an average vertical force of 1000 N against a barbell while the barbell moves 0.8 m downward

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Calculate Work Table of Variables Force = N Displacement = -0.8 m Force is positive due to pushing upward Displacement is negative due to movement downward

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Calculate Work Table of Variables Force = N Displacement = -0.8 m Select the equation and solve:

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Contemplate lDuring negative work on the bar, what is the dominant type of activity (contraction) occurring in the muscles? lWhen positive work is being performed on the bar?

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EMG during the Bench Press On elbow

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Extra Practice on KE

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Calculate Kinetic Energy How much KE possessed by a 150 pound female volleyball player moving downward at 3.2 m/s after a block?

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Calculate Kinetic Energy Table of Variables l150 lbs = kg of mass l-3.2 m/s Select the equation and solve: KE = ½ m v 2 lKE = ½ (68.18 kg)(-3.2 m/s) 2 lKE = ½ (68.18 kg)(10.24 m/s/s) lKE = ½ ( kg m/s/s) lKE = Nm or J

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