Work, Power & Energy

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

2. 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)

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 • 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?

5. 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

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

7. 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 = 130 Nm (Joules) • Multiply by the number of steps

8. Work on a stair stepper • Work = Fd • Force • Push on the step • ???? • Displacement • Step Height • 8 inches • “Work” per step • ???N x .2m = ???Nm (Joules)

9. 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

10. Kinetic Energy Energy due to motion reflects • the mass • the velocity of the object KE = 1/2 mv2

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  0.145 kg Velocity = 35.8 m/s

13. Calculate Kinetic Energy Table of Variables Mass = 145 g  0.145 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)(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

14. Gravitational PE • Affected by the object’s • weight • mg • elevation (height) above reference point • ground or some other surface • h 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. GPE relative to mat Table of Variables m = 45 kg g = -9.81 m/s/s h = 4 m PE = mgh PE = 45kg * -9.81 m/s/s * 4 m PE = - 1765.8 J Calculate GPE

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

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

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

20. Extension…

21. Power • The rate of doing work • Work = Fd Units: Fd/s = J/s = watt

22. 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

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

24. Power on a cycle ergometer • Work = Fd • Force: 3kg • Displacement: 6m /rev • “Work” per revolution • 3kg x 6 m = 18 kgm • 60 rev/min

25. 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

26. 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

27. 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

28. 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

29. - & + 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

30. - & + 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

31. 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

32. 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

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

34. 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?

35. EMG during the Bench Press 180 90 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 • 150 lbs = 68.18 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 (698.16 kg m/s/s) • KE = 349.08 Nm or J