1. 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

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
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 = Nm
Multiply by the number of steps

8. 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

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. KE = 1/2 mv2 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  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 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

14. 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?)

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
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. 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
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 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 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 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 = 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