1. Explaining the Causes of Motion in a Different Way
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) 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. Simple machines and work-- Forces involved:
Input Force FI
Force applied to a machine
Output Force FO
Force applied by a machine

9. Two forces, thus two types of work
Work Input
work done on a machine
=Input force x the distance through which that force acts (input distance)
Work Output
Work done by a machine
=Output force x the distance through which the resistance moves (output distance)

10. Mechanical Advantage (MA) – expressed in a ratio WITH NO UNITS!!
The number of times a machine multiplies the input force.
MA = output force/input force

11. Efficiency Efficiency can never be greater than 100 %. Why?
Some work is always needed to overcome friction.
A percentage comparison of work output to work input.
work output (WO) / work input (WI)

12. Mechanical advantage of levers.
Ideal = input arm length/output arm length
input arm = distance from input force to the fulcrum
output arm = distance from output force to the fulcrum

13. 2. The Wheel and Axle A lever that rotates in a circle.
A combination of two wheels of different sizes.
Smaller wheel is termed the axle.
IMA = radius of wheel/radius of axle.

14. 3. The Inclined Plane A slanted surface used to raise an object.
Examples: ramps, stairs, ladders
IMA = length of ramp/height of ramp
Can never be less than one.

15. Energy Energy (E) is defined as the capacity to do work (scalar)
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

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

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

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

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

20. 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

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

22. 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

23. 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

24. Calculate Kinetic Energy
Table of Variables
m = 100 Kg
v = 4.0 m/s
Select the equation and solve:
KE = ½ m v2
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 v2
KE = ½ (175)(3.75)2
KE = 1230 Nm or J

25. Calculate Momentum 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 =

26. 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

27. 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?)

28. 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

29. 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

30. Calculate GPE More on this 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

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

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
What work was performed on the bar by lifter?
Fd =  KE +  PE
Fd = ½ m(vf –vi)2 + mgh
Fd = 100kg * - 10 m/s/s * 0.75 m
Fd = 750 J
W = Fd
W = 100 kg * .75m
W = 75 kg m
W = 75 kg m (10) = 750 J

35. Work - Energy Relationship
What is GPE at the start & end of the press?
End (ascent)
PE = mgh
PE = 100 kg * -10 m/s/s * 1.50 m
PE = 1500 J
Start (ascent)
PE = 100 kg * -10 m/s/s * 0.75m
PE = 750 J

36. Work - Energy Relationship
Of critical importance
Sport and exercise =  velocity
increasing and decreasing kinetic energy of a body
similar to the impulse-momentum relationship
Ft = m (vf-vi)

37. 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

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

39. 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

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