1. 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)

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) 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
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

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

7. - & + 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

8. - & + 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

9. 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?

10. Calculate Work Table of Variables Force = +1000 N
Displacement = -0.8 m

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

12. - & + 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

13. 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?

14. EMG during the Bench Press

15. 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

16. 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

17. 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

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

19. Kinetic Energy Units: reflect the units of mass * v2
Units KE = Units work

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

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

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

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

24. 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

25. 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

26. 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?)

27. 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

28. 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

29. Calculate GPE GPE relative to mat Table of Variables m = 45 kg
g = m/s/s
h = 4 m
GPE relative to ground
Table of Variables
m = 45 kg
g = m/s/s
h = 5.25 m

30. 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 kx2

31. 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 = n /m x = m (7 mm), Achilles tendon in walking
When a floor/shoe sole is deformed, energy is stored in the material .

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

33. 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?

34. Work - Energy Relationship
Of critical importance
Sport and exercise =  velocity
increasing and decreasing kinetic energy of a body
similar to the impulse-momentum relationship

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

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

37. 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

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

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

40. 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??)

41. 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

42. 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

43. 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