# Chapter 4 Explaining the Causes of Motion in a Different Way

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Chapter 4 Explaining the Causes of Motion in a Different Way
Work, Power & Energy Chapter 4 Explaining the Causes of Motion in a Different Way Day one- work and force relationship notes and problems (Solve for D, F…) (make problems sheet) Two- lab (yellow) Three- work and energy- notes and problems (blue sheet) Day Four- review (lab?) Day five- QUIZ

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 Force is positive due to pushing upward Displacement is positive due to moving upward

Calculate Work Table of Variables: Force = +1000 N
Displacement = +0.8 m Select the equation and solve:

Work performed climbing stairs
Work = Fd Force Subject weight From mass, ie 65 kg Displacement Height of each step Typical 8 inches (20cm) Work per step 650N x 0.2 m = Nm Multiply by the number of steps

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 (scalar)
Many forms No more created, only converted chemical, sound, heat, nuclear, mechanical Mechanical Energy Kinetic Energy (KE): energy due to motion Potential Energy (PE): energy due to position

KE = 1/2 mv2 Kinetic Energy Energy 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 Variables Mass = 145 g  kg Velocity = 35.8 m/s

Calculate Kinetic Energy
Table of Variables Mass = 145 g  kg Velocity = 35.8 m/s Select the equation and solve: KE = ½ m v2 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

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?)

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 kg
g = m/s/s h = 4 m PE = mgh PE = 45kg * m/s/s * 4 m PE = 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

Work - Energy Relationship
If more work is done, greater energy greater average force greater displacement

Extension…

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

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 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 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 m step rate 56.9 /min vs 28.8 /min Results: VO2 similar fast/short steps vs slow/deep steps

- & + 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 a ball push off (upward) phase of a jump

- & + 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

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

Calculate Work Table of Variables Force = +1000 N
Displacement = -0.8 m Force is positive due to pushing upward Displacement is negative due to movement downward

Calculate Work Table of Variables Force = +1000 N
Displacement = -0.8 m Select the equation and solve:

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
180 90 On elbow

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 = kg of mass -3.2 m/s Select 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 ( kg m/s/s) KE = Nm or J

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