Presentation is loading. Please wait.

Presentation is loading. Please wait.

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

Similar presentations


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 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

8 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

9 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

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

11 - & + 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

12 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?

13 EMG during the Bench Press On elbow

14 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

15 Work on a stair stepper lWork = Fd lForce l Push on the step l???? lDisplacement l Step Height l8 inches l“Work” per step l ???N x.203 m = ???Nm

16 Work on a cycle ergometer lWork = Fd lForce l belt friction on the flywheel lmass (eg 3 kg) lDisplacement l revolution of the pedals lMonark: 6 m l“Work” per revolution

17 Work on a cycle ergometer lWork = Fd lForce l belt friction on the flywheel lmass (eg 3 kg) lDisplacement l revolution of the pedals lMonark: 6 m l“Work” per revolution l 3kg x 6 m = 18 kgm

18 Similar principle for wheelchair

19 …and for handcycling ergometer

20 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 lKinetic Energy (KE): l energy due to motion lPotential Energy (PE): l energy due to position or deformation

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

22 Kinetic Energy Units: reflect the units of mass * v 2 lUnits KE = Units work

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

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

25 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

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

27 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

28 Calculate Kinetic Energy Calculate Kinetic Energy Compare KE possessed by: l a 220 pound (100 kg) running back moving forward at 4.0 m/s l a 385 pound (175 kg) lineman moving forward at 3.75 m/s Bonus: calculate the momentum of each player

29 Calculate Kinetic Energy Calculate Kinetic Energy Table of Variables m = 100 Kg v = 4.0 m/s Select the equation and solve: KE = ½ m v 2 KE = ½ (100 kg)(4.0 m/s) 2 KE = 800 Nm or J Table of Variables m = 175 kg v = 3.75 m/s Select the equation and solve: KE = ½ m v 2 KE = ½ (175)(3.75) 2 KE = 1230 Nm or J

30 omentum Calculate Momentum Calculate M Momentum = mass times velocity Player 1 = 100 kg * 4.0 m/s Player 1 = 400 kg m/s Player 2 = 175 * 3.75 m/s Player 2 =

31 Potential Energy Two forms of PE: lGravitational PE: l energy due to an object’s position relative to the earth lStrain PE: l due to the deformation of an object

32 Gravitational PE lAffected by the object’s l weight lmg l elevation (height) above reference point l ground or some other surface lh GPE = mgh Units = Nm or J (why?)

33 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

34 Calculate GPE 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

35 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 GPE relative to ground Table of Variables m = 45 kg g = m/s/s h = 5.25 m PE = mgh PE = 45m * m/s/s * 5.25 m PE = J More on this

36 Conversion of KE to GPE and GPE to KE and KE to GPE and …

37 Strain PE Affected by the object’s lamount of deformation l greater deformation = greater SE l  x 2 = change in length or deformation of the object from its undeformed position lstiffness l resistance to being deformed l k = stiffness or spring constant of material SE = 1/2 k  x 2

38 Strain Energy lWhen a fiberglass vaulting pole bends, strain energy is stored in the bent pole Pole vault explosion

39 Strain Energy lWhen a fiberglass vaulting pole bends, strain energy is stored in the bent pole lBungee jumping

40 Strain Energy lWhen a fiberglass vaulting pole bends, strain energy is stored in the bent pole lBungee jumping lHockey sticks

41 Strain Energy lWhen a fiberglass vaulting pole bends, strain energy is stored in the bent pole lBungee jumping lWhen a tendon/ligament/muscle is stretched, strain energy is stored in the elongated elastin fibers (Fukunaga et al, 2001, ref#5332 ) l k = n /m x = m (7 mm), Achilles tendon in walking lWhen a floor/shoe sole is deformed, energy is stored in the material. Plyometrics

42 Work - Energy Relationship lThe work done by an external force acting on an object causes a change in the mechanical energy of the object

43 Work - Energy Relationship lThe work done by an external force acting on an object causes a change in the mechanical energy of the object l Bench press ascent phase linitial position = 0.75 m; velocity = 0 lfinal position = 1.50 m; velocity = 0 lm = 100 kg lg = -10 m/s/s lWhat work was performed on the bar by lifter? lWhat is GPE at the start & end of the press?

44 Work - Energy Relationship lWhat work was performed on the bar by lifter? lFd =  KE +  PE lFd = ½ m(v f –v i ) 2 + mgh lFd = 100kg * - 10 m/s/s * 0.75 m lFd = 750 J lW = Fd lW = 100 kg *.75m lW = 75 kg m lW = 75 kg m (10) = 750 J

45 Work - Energy Relationship lWhat is GPE at the start & end of the press? lEnd (ascent) lPE = mgh lPE = 100 kg * -10 m/s/s * 1.50 m lPE = 1500 J lStart (ascent) lPE = 100 kg * -10 m/s/s * 0.75m lPE = 750 J

46 Work - Energy Relationship lOf critical importance lSport and exercise =  velocity l increasing and decreasing kinetic energy of a body l similar to the impulse-momentum relationship Ft = m (v f -v i )

47 Work - Energy Relationship lIf more work is done, greater  energy l greater average force l greater displacement lEx. Shot put technique ( ). lIf displacement is restricted, average force is __________ ? (increased/decreased) l “giving” with the ball l landing hard vs soft

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

49 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

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

51 Power on a cycle ergometer lWork = Fd lForce: 3kg lDisplacement: 6m /rev l“Work” per revolution l 3kg x 6 m = 18 kgm l60 rev/min

52 Power on a cycle ergometer lWork = Fd lForce: 3kg lDisplacement: 6m /rev l“Work” per revolution l 3kg x 6 m = 18 kgm l60 rev/min 1 Watt = 6.12 kgm/min

53 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

54 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

55 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


Download ppt "Work, Power & Energy Work, Power & Energy Chapter 4 Explaining the Causes of Motion in a Different Way."

Similar presentations


Ads by Google