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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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Presentation on theme: "Work, Power & Energy Work, Power & Energy Chapter 4 Explaining the Causes of Motion in a Different Way."— Presentation transcript:

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

2 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

3 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

4 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?

5 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

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

7 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

8 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

9 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

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

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

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

13 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

14 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?)

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

16 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

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

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

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

20 Extension…

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

22 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

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

24 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

25 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

26 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

27 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

28 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

29 - & + 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

30 - & + 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

31 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

32 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

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

34 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?

35 EMG during the Bench Press On elbow

36 Extra Practice on KE

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

38 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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