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Work, Energy Power rev.pptbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbn
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: Newton•meter (Joule)
ft . lbs (horsepower)
nt
displacem e
F orce
W ork
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:
J
Joule
Nm
Work
m
N
Work
nt
displaceme
Force
Work
800
800
800
8
.
0
1000
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 a ball
push off (upward) phase of a jump
8. 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
9. 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
10. Calculate Work
Table of Variables
Force = +1000 N
Displacement = -0.8 m
Select the equation and solve:
J
Joule
Nm
Work
m
N
Work
nt
displaceme
Force
Work
800
800
800
8
.
0
1000
11. - & + 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
12. 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 = 130.0 Nm
Multiply by the number of steps
13. Work on a stair stepper
Work = Fd
Force
Push on the step
????
Displacement
Step Height
8 inches
“Work” per step
???N x .203 m = ???Nm
14. Work on a cycle ergometer
Work = Fd
Force
belt friction on the flywheel
mass (eg 3 kg)
Displacement
revolution of the pedals
Monark: 6 m
“Work” per revolution
15. Work on a cycle ergometer
Work = Fd
Force
belt friction on the flywheel
mass (eg 3 kg)
Displacement
revolution of the pedals
Monark: 6 m
“Work” per revolution
3kg x 6 m = 18 kgm
18. Power
The rate of doing work
Work = Fd
Units: Fd/s = J/s = watt
velocity
Force
Power
t
Fd
Power
time
Work
Power
/
/
19. 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
20. Calculate & compare power
Lifter A
Table of Variables
F = 1000 N
d = 0.8 m
t = 0.50 s
Lifter B’s time would
be .75 sec instead
of .5 sec
w
s
J
Power
s
m
N
Power
t
Fd
Power
1600
50
.
0
800
50
.
0
8
.
0
1000
21.
22. 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
26. 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
27. Calculate Kinetic Energy
How much KE possessed by
a 68.1 kg female volleyball
player moving downward at
3.2 m/s after a block?
28. Calculate Kinetic Energy
Table of Variables
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
29. 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
30. 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
31. Potential Energy
Two forms of PE:
Gravitational PE:
energy due to an object’s position
relative to the earth
Strain PE:
due to the deformation of an
object
32. Gravitational PE
Affected by the object’s
weight
mg
elevation (height) above reference point
ground or some other surface
h
GPE = mgh
33. 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
34. Calculate GPE
GPE relative to mat
Table of Variables
m = 45 kg
g = 10 m/s/s
h = 4 m
PE = mgh
PE = 45kg * -9.81
m/s/s * 4 m
PE = 1765.8 J
GPE relative to ground
Table of Variables
m = 45 kg
g = 10m/s/s
h = 5.25 m
PE = mgh
PE = 45kg * -9.81
m/s/s * 5.25 m
PE = 2317.6 J
36. Strain Energy
When a fiberglass vaulting pole
bends, strain energy is stored in
the bent pole
Bungee jumping
When a tendon/ligament/muscle is
stretched, strain energy is stored
in the elongated elastin fibers
.
37. Work - Energy Relationship
The work done by an external force
acting on an object causes a change in
the mechanical energy of the object
)
(
2
1 2
i
f
i
f r
r
mg
v
v
m
Fd
PE
KE
Fd
Energy
Fd