Work, Power & Energy
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Transcript of Work, Power & Energy
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Work, Power & Energy
Explaining the Causes of Motion in a Different Way
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WorkThe product of force and the amount
of displacement along the line of action of that force.
Units: ft . lbs (horsepower) Newton•meter (Joule)
ntdisplacemeForceWork
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Work = F x dTo calculate work done on an object, we
need:The ForceThe average magnitude of the force The direction of the forceThe DisplacementThe magnitude of the change of positionThe direction of the change of position
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Calculate WorkDuring 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
How much work did the lifter do to the barbell?
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Calculate WorkTable of Variables:Force = +1000 NDisplacement = +0.8 m
Force is positive due to pushing upwardDisplacement is positive due to moving
upward
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Calculate WorkTable of Variables:Force = +1000 NDisplacement = +0.8 mSelect the equation and solve:
JJouleNmWork
mNWorkntdisplacemeForceWork
8008008008.01000
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Work performed climbing stairs Work = Fd Force
Subject weightFrom mass, ie 65 kg
Displacement Height of each step
Typical 8 inches (20cm) Work per step
650N x 0.2 m = 130 Nm (Joules) Multiply by the number of steps
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Work on a stair stepperWork = FdForce
Push on the step????
Displacement Step Height
8 inches“Work” per step
???N x .2m = ???Nm (Joules)
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Energy Energy (E) is defined as the capacity to do
work (scalar) Many forms
No more created, only convertedchemical, sound, heat, nuclear, mechanical
Mechanical Energy Kinetic Energy (KE):
energy due to motion Potential Energy (PE):
energy due to position
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Kinetic EnergyEnergy due to motion reflects
the mass the velocity
of the object
KE = 1/2 mv2
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Calculate Kinetic EnergyHow much KE in a 5 ounce baseball (145 g) thrown at 80 miles/hr (35.8 m/s)?
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Calculate Kinetic EnergyTable of VariablesMass = 145 g 0.145 kgVelocity = 35.8 m/s
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Calculate Kinetic EnergyTable of VariablesMass = 145 g 0.145 kgVelocity = 35.8 m/sSelect the equation and solve:KE = ½ m v2
KE = ½ (0.145 kg)(35.8 m/s)2
KE = ½ (0.145 kg)(1281.54 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 PEAffected by the object’s
weight mg
elevation (height) above reference point ground or some other surfaceh
GPE = mgh
Units = Nm or J (why?)
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Calculate GPEHow much gravitational potential
energy in a 45 kg gymnast when she is 4m above the mat of the trampoline?
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Calculate GPEGPE relative to mat Table of Variables m = 45 kgg = -9.81 m/s/sh = 4 mPE = mghPE = 45kg * -9.81 m/s/s * 4 mPE = - 1765.8 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 RelationshipWork is the change in the
mechanical energy of the object
Fd KineticEnergyFd KE
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Work - Energy Relationship If more work is done, greater
energy greater average forcegreater displacement
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Extension…
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PowerThe rate of doing work
Work = Fd
Units: Fd/s = J/s = wattvelocityForcePower
tFdPowertimeWorkPower
//
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Calculate & compare powerDuring 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
Lifter A: 0.50 secondsLifter B: 0.75 seconds
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Calculate & compare powerLifter ATable of VariablesF = 1000 Nd = 0.8 mt = 0.50 s
Lifter B
wsJPower
smNPower
tFdPower
160050.0
80050.0
8.01000
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Power on a cycle ergometer Work = Fd Force: 3kg Displacement: 6m /rev “Work” per revolution
3kg x 6 m = 18 kgm 60 rev/min
min/1080""min/6018""
min/""/""
kgmPowerkgmPowerrevFdPowertFdPower
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Power on a cycle ergometer Work = Fd Force: 3kg Displacement: 6m /rev “Work” per revolution
3kg x 6 m = 18 kgm 60 rev/min
min/1080""min/6018""
min/""/""
kgmPowerkgmPowerrevFdPowertFdPower
1 Watt = 6.12 kgm/min
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Compare “power” in typical stair stepping
Work = Fd Force: Push on the step
constant setting Displacement
Step Height: 5” vs 10” 0.127 m vs 0.254 m
step rate 56.9 /min vs 28.8 /min
Time per step60s/step rate
Thesis data from Nikki Gegel and Michelle Molnar
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Compare “power” in typical stair stepping
Work = Fd Force: Push on the step
constant setting Displacement
Step Height: 5” vs 10” 0.127 m vs 0.254 m
step rate 56.9 /min vs 28.8 /min
)08.2/254(.)05.1/127(.
10
5
smFPowersmFPower
vFPower
inch
inch
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Compare “power” in typical stair stepping
Work = Fd Force: Push on the step
constant setting Displacement
Step Height: 5” vs 10” 0.127 m vs 0.254 m
step rate 56.9 /min vs 28.8 /min
smFPowersmFPower
vFPower
inch
inch
/122.0/121.0
10
5
Results: VO2 similar fast/short steps vs slow/deep steps
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- & + WorkPositive work is performed
when the direction of the force and the direction of motion are the sameascent phase of the bench pressThrowing a ballpush off (upward) phase of a jump
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- & + WorkPositive work Negative work is performed
when the direction of the force and the direction of motion are the oppositedescent phase of the bench presscatching landing phase of a jump
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Calculate WorkDuring 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 WorkTable of VariablesForce = +1000 NDisplacement = -0.8 m
Force is positive due to pushing upwardDisplacement is negative due to
movement downward
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Calculate WorkTable of VariablesForce = +1000 NDisplacement = -0.8 mSelect the equation and solve:
JJouleNmWork
mNWorkntdisplacemeForceWork
8008008008.01000
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ContemplateDuring negative work on the bar,
what is the dominant type of activity (contraction) occurring in the muscles?
When positive work is being performed on the bar?
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EMG during the Bench Press
On elbow180
90
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Extra Practice on KE
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Calculate Kinetic EnergyHow 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 EnergyTable of Variables 150 lbs = 68.18 kg of mass -3.2 m/sSelect the equation and solve:KE = ½ m v2
KE = ½ (68.18 kg)(-3.2 m/s)2
KE = ½ (68.18 kg)(10.24 m/s/s) KE = ½ (698.16 kg m/s/s) KE = 349.08 Nm or J