Transcript Chapter 6

Chapter 6
Linear Kinetics
Explaining the
Causes of
Linear Motion
Review
Newton’s
????
Laws
of
Motion
Newton I: Law of Inertia
A body will maintain its current state
of motion unless acted on by an
unbalanced external force
• External forces include airresistance and friction
• Examples
• passenger in car
• projectile motion (horizontal)
Newton II: Law of Acceleration
F = ma
• A force applied to a body causes an acceleration
of the body of a magnitude proportional to the
force, in the direction of the force, and
inversely proportional to the body mass
• Example, kicking a soccer ball
• At an instant in time
Newton III: Law of
Reaction
•For every action there is
an equal and opposite
reaction (forces act in
mirrored pairs)
•Ground reaction forces
How
measurement
has
changed.
Current state of motion
• How fast and in what direction
• velocity: a vector quantity
• How resistant to changing state of motion
• inertia: quantified by mass
Momentum: mass x velocity
Momentum = mv
Momentum
• Newton I: Law of inertia
• Principal of Conservation of
Momentum
• In the absence of external
forces, the total momentum of
a given system remains
constant
Momentum Example
(15 July 1999, Alabama) A 25-year-old soldier died of
injuries sustained from a 3-story fall, precipitated by
his attempt to spit farther than his buddy. His plan
was to hurl himself towards a metal guardrail while
expectorating, in order to add momentum to his
saliva. In a tragic miscalculation, his momentum
carried him right over the railing, which he caught
hold of for a few moments before his grip slipped,
sending him plummeting 24 feet to the cement below.
The military specialist had a blood alcohol content of
0.14%, impairing his judgment and paving the way for
his opportunity to win a Darwin Award.
Newton II: Law of Acceleration
F = ma
Develop the impulse-momentum
relationship from this equation
Impulse to increase momentum
and decrease momentum
• Mechanical objective of human performance
• move at a particular speed in a particular direction
• ===> momentum
• Alter perfomance (momentum)
• force: magnitude, direction, point of application,
line of action
• time: duration of force application
Vertical jump
• Jump
• increase momentum to some high value
• rest to upward motion
• Certain impulse is required
•
m (vv - vi)
• Alter time of force application
• short time, large force
• long time, small force
Soft and stiff landings
• Landing
• decrease momentum to Zero
• downward motion to rest
• Certain impulse is required
•
m (vv - vi) same with landing from a height
• Alter time of force application
• short time, large force
• long time, small force
Child is uninjured
because:
1) soft ground and
leaves increase time
over which child stops
Effect on Force???
Homework Assignment
• Due one week from today
• unless otherwise stated
• Review Questions Chapter 6
• pages 111-112
Role of Movement Analyst
Bob Norman (1976) & Shirl Hoffman (1977)
• Recognize symptoms of malperformance
• Is performance acceptable? Is the goal attained?
• Process vs Product
• Trouble-shoot the cause of the malperformance
• Critical ability lacking
• Skill performance deficiency
• Psychosocial problem
• Prescribe cues to correct malperformance
• Make the appropriate intervention
Critical ability lacking
Mismatch between the individual
& the environment
Modify the environment
Adapt the environment to the individual to enhance
chances of success.
Psychosocial problems.
Psychosocial problems.
I joined a health club last year, spent about $400.
Haven’t lost a pound.
Apparently, you have to show up.
Psychosocial
problems.
Skill Performance Deficiency
• Mechanical objective of a skill
• momentum in particular direction(s)
Bob Norman (1976)
• Mechanical objective of a skill
• Idiosyncracy
• individual nuance of performance that has no
direct influence on mechanics of performance
• Michael Jordan
• batting stance
Unwarranted
focus
on static
position of
batting stance
So prove it.
Physics is not the only science important in the game of baseball;
it’s also a game of psychology, of pitchers trying to stare down
batters, and batters trying to intimidate pitchers.
And no matter what you tell me, I don’t believe that some guy
standing in the batter’s box with his bat dangling loosely from his
hands has nearly as much power as a guy snarling and gripping the
bat hard, waving it menacingly over his head.
It may be good science, but I’ll bet the second guy gets more hits.
Suzanne Sparacino, Commack NY
Letter to the editor, Discover magazine.
Bob Norman (1976)
• Mechanical objective of a skill
• Idiosyncracy
• Symptom of an error
• what we see go wrong (kinematics)
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•
•
•
not high enough
not fast enough
wrong direction
body positions
Swing
Level?
Paul Popovich
March 29, 1993.
Say
what
you
mean.
Follow
through
Symptom
or
Cause?
Game-winning
shot vs Utah Jazz
19**???
Lead
with
the
elbow
Shoulder
External
Rotation:
Symptom
or
Cause
of
Great
Performance?
Bob Norman (1976)
•
•
•
•
Mechanical objective of a skill
Idiosyncracy
Symptom of an error
Cause of an error
• the underlying reason for the poor performance
• mechanically: error in impulse application
Mechanical Phases
of skill performance
• Ritual Phase
• Full of idiosyncrasies, useful for mental focus
• Preparation
• Wind up
• Storage of elastic energy, increase ROM (Benefits?)
• Execution
• Acceleration
• Apply impulse to the body in the desired direction
• Follow through
• Bring moving parts to rest
• Safety, symptom of good performance
Volleyball Underhand Serve
Peer Rating Evaluation Sheet
from a middle school session at IAHPERD conference
1.
2.
3.
4.
5.
6.
7.
8.
9.
Is the arm which is holding the ball across the front of the body?
Is the ball held at or below the waist?
Is the body bent at the waist before the person hits the ball?
Is the striking arm brought back with a straight elbow?
Is the ball hit from a short toss or out of the hand?
Does the hitting arm swing from the shoulder?
Does the hitting arm swing straight, not across the body?
Is the person stepping or standing with opposition?
Is there a weight shift from the rear foot to the front foot when
the ball is hit?
10. Does the striking hand follow through at least as high as the head
after the ball is hit?
Possible reasons for elderly to
walk slower
• Not willing to walk faster
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•
•
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Wise enough to know that walking faster changes nothing
Careful stepping due to slowing of reflexes
Cautious
Afraid of road hazards
Afraid of falls due to OP, other catastrophes
• Not able to walk faster
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•
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•
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Reduced ability to maintain dynamic balance
Reduced ROMs
Reduced contents of fast-twitch muscle fiber
Joint or muscle pathologies
Reduce CV fitness
Long Lies
• Phenomenom
• Individual cannot rise from the ground
• After a fall
• Gardening
• Housecare
• Why not? List possible reasons, according to the Hoffman
Checklist
• Importance: Develop an intervention for prevention
• Strength training?
• Flexibility training?
• Technique Training
• What technique? Old vs Young? Healthy vs Infirm?
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