work, power & energy work, power & energy chapter 4 explaining the causes of motion in a...
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Work, Power & Energy
Chapter 4
Explaining the Causes of Motion in a Different Way
WorkThe product of force and the amount
of displacement along the line of action of that force.
Units: ft . lbs (horsepower) Newton•meter (Joule) e
ntdisplacemeForceWork
Work = F x d
To 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
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?
Calculate Work
Table of Variables:Force = +1000 NDisplacement = +0.8 m
Force is positive due to pushing upwardDisplacement is positive due to moving
upward
Calculate Work
Table of Variables:Force = +1000 NDisplacement = +0.8 mSelect the equation and solve:
JJouleNmWork
mNWork
ntdisplacemeForceWork
800800800
8.01000
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 = 130.0 Nm
Multiply by the number of steps
Work on a stair stepper
Work = FdForce
Push on the step????
Displacement Step Height
8 inches
“Work” per step???N x .203 m = ???Nm
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
Kinetic EnergyEnergy due to motion reflects
the mass the velocity
of the object
KE = 1/2 mv2
Calculate Kinetic Energy
How much KE in a 5 ounce baseball (145 g) thrown at 80 miles/hr (35.8 m/s)?
Calculate Kinetic Energy
Table of VariablesMass = 145 g 0.145 kgVelocity = 35.8 m/s
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
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?)
Calculate GPE
How much gravitational potential energy in a 45 kg gymnast when she is 4m above the mat of the trampoline?
Calculate GPE
GPE 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
Conversion of KE to GPE and GPE to KE and KE to GPE and …
Work - Energy Relationship
Work is the change in the mechanical energy of the object
Fd KineticEnergy
Fd KE
Work - Energy Relationship
If more work is done, greater energy greater average forcegreater displacement
Extension…
PowerThe rate of doing work
Work = Fd
Units: Fd/s = J/s = watt
velocityForcePower
tFdPower
timeWorkPower
/
/
Calculate & compare power
During 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
Calculate & compare power
Lifter ATable of VariablesF = 1000 Nd = 0.8 mt = 0.50 s
Lifter B
ws
JPower
s
mNPower
t
FdPower
160050.0
80050.0
8.01000
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/""
/""
kgmPower
kgmPower
revFdPower
tFdPower
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/""
/""
kgmPower
kgmPower
revFdPower
tFdPower
1 Watt = 6.12 kgm/min
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
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
smFPower
smFPower
vFPower
inch
inch
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
smFPower
smFPower
vFPower
inch
inch
/122.0
/121.0
10
5
Results: VO2 similar fast/short steps vs slow/deep steps
- & + 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
- & + 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
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
Calculate Work
Table of VariablesForce = +1000 NDisplacement = -0.8 m
Force is positive due to pushing upwardDisplacement is negative due to
movement downward
Calculate Work
Table of VariablesForce = +1000 NDisplacement = -0.8 mSelect the equation and solve:
JJouleNmWork
mNWork
ntdisplacemeForceWork
800800800
8.01000
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?
EMG during the Bench Press
On elbow180
90
Extra Practice on KE
Calculate Kinetic Energy
How much KE possessed by a 150 pound female volleyball player moving downward at 3.2 m/s after a block?
Calculate Kinetic Energy
Table 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
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