Chapter 7 Explaining the Causes of Motion Without Newton (sort of) Work, Power & Energy Chapter 7 Explaining the Causes of Motion Without Newton (sort of)
Work The product of force and the amount of displacement along the line of action of that force. Units: ft . lbs (horsepower) Newton•meter (Joule) e
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
Calculate Work During 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 N Displacement = +0.8 m
Calculate Work Table of Variables: Force = +1000 N Displacement = +0.8 m Select the equation and solve:
- & + Work Positive work is performed when the direction of the force and the direction of motion are the same ascent phase of the bench press throwing push off phase of a jump
- & + Work Positive work is performed when the direction of the force and the direction of motion are the same Negative work is performed when the direction of the force and the direction of motion are the opposite
Calculate Work During 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 How much work did the lifter do to the barbell?
Calculate Work Table of Variables Force = +1000 N Displacement = -0.8 m
Calculate Work Table of Variables Force = +1000 N Displacement = -0.8 m Select the equation and solve:
- & + Work Positive work Negative work is performed when the direction of the force and the direction of motion are the opposite descent phase of the bench press catching landing phase of a jump
Contemplate During 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
Work on a cycle ergometer Work = Fd Force belt friction on the flywheel mass ie 3 kg Displacement revolution of the pedals Monark: 6 m “Work” per revolution 3kg x 6 m = 18 kgm
Work on a stair stepper Work = Fd Force Displacement “Work” per step 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 Many forms No more created, only converted chemical, sound, heat, nuclear, mechanical Kinetic Energy (KE): energy due to motion Potential Energy (PE): energy due to position or deformation
KE = 1/2 mv2 Kinetic Energy Energy due to motion reflects the mass the velocity of the object KE = 1/2 mv2
Kinetic Energy Units: reflect the units of mass * v2 Units KE = Units work
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 Variables Mass = 145 g 0.145 kg Velocity = 35.8 m/s
Calculate Kinetic Energy Table of Variables Mass = 145 g 0.145 kg Velocity = 35.8 m/s Select the equation and solve:
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 Compare KE possessed by: a 220 pound (100 kg) running back moving forward at 4.0 m/s a 385 pound (175 kg) lineman moving forward at 3.75 m/s Bonus: calculate the momentum of each player
Potential Energy Two forms of PE: Gravitational PE: Strain PE: energy due to an object’s position relative to the earth Strain PE: due to the deformation of an object
Gravitational PE Affected by the object’s GPE = mgh weight mg elevation (height) above reference point ground or some other surface h GPE = mgh Units = Nm or J (why?)
Take a look at the energetics of a roller coaster Calculate GPE How much gravitational potential energy in a 45 kg gymnast when she is 4m above the mat of the trampoline? Take a look at the energetics of a roller coaster
Calculate GPE Trampoline mat is 1.25 m above the ground How much gravitational potential energy in a 45 kg gymnast when she is 4m above the mat of the trampoline? Trampoline mat is 1.25 m above the ground
Calculate GPE GPE relative to mat Table of Variables m = 45 kg g = -9.81 m/s/s h = 4 m GPE relative to ground Table of Variables m = 45 kg g = -9.81 m/s/s h = 5.25 m
Strain PE Affected by the object’s amount of deformation stiffness greater deformation = greater SE x2 = change in length or deformation of the object from its undeformed position stiffness resistance to being deformed k = stiffness or spring constant of material SE = 1/2 kx2
Strain Energy When a fiberglass vaulting pole bends, strain energy is stored in the bent pole When a tendon/ligament/muscle is stretched, strain energy is stored in the elongated elastin fibers (Fukunaga et al, 2001, ref#5332) k = 10000 n /m x = 0.007 m (7 mm), Achilles tendon in walking When a floor/shoe sole is deformed, energy is stored in the material .
Work - Energy Relationship The work done by an external force acting on an object causes a change in the mechanical energy of the object
Work - Energy Relationship The work done by an external force acting on an object causes a change in the mechanical energy of the object Bench press ascent phase initial position = 0.75 m; velocity = 0 final position = 1.50 m; velocity = 0 m = 100 kg g = -10 m/s/s What work was performed on the bar by lifter? What is GPE at the start & end of the press?
Work - Energy Relationship Of critical importance Sport and exercise = velocity increasing and decreasing kinetic energy of a body similar to the impulse-momentum relationship
Work - Energy Relationship If more work is done, greater energy greater average force greater displacement Ex. Shot put technique (121-122). If displacement is restricted, average force is __________ ? (increased/decreased) “giving” with the ball landing hard vs soft
Power The rate of doing work Work = Fd Units: Fd/s = J/s = watt
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 seconds Lifter B: 0.75 seconds
Calculate & compare power Lifter A Table of Variables F = 1000 N d = 0.8 m t = 0.50 s Lifter B
Power on a cycle ergometer Work = Fd Force: 3kg Displacement: 6m /rev “Work” per revolution 3kg x 6 m = 18 kgm 60 rev/min
Power on a cycle ergometer Work = Fd Force: 3kg Displacement: 6m /rev “Work” per revolution 3kg x 6 m = 18 kgm 60 rev/min 1 Watt = 6.12 kgm/min (How so??)
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 step 60s/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
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 Results: VO2 similar fast/short steps vs slow/deep steps