Download How Do I Move? Chapter 7 Biomechanics Centre of Mass

How Do I Move?
Chapter 7
The Science of Biomechanics
Biomechanics
Examines the internal and external
forces acting on the human body
and the effects produced by these
forces
Aids in technique analysis and the
development of innovative
equipment designs
Draws on knowledge from sports
medicine, physical therapy,
kinesiology, and biomechanical
engineering
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Centre of Mass
Centre of mass
outside of body
Located at the balance point of
a body; a point found in or
about a body where the mass
could be concentrated
Generally, 15 cm above the
crotch, or approximately 55%
of standing height in females
and 57% in males
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The Laws of Motion
Newton’s Three Laws of Motion:
Describe the relationship between all the external forces
acting on the human body at any time and the resulting
motion of the total body
Sir Isaac Newton developed these laws to explain why
things move the way they do
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Newton’s First Law: Inertia
An object will not change its state of motion (it will
continue to be at rest or moving with constant velocity),
unless acted upon by a net , external force
For example: because of their large mass, football linemen
are difficult to move out of the way
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Newton’s Second Law: Acceleration
For linear movements, the acceleration (a) a body
experiences is proportional to the force (F) causing it, and
takes place in the same direction as the force
F = m.a
where m is the mass of the body
For angular movements, the angular acceleration of a
body is proportional to the moment of force causing it, and
takes place in the same direction as the moment of force
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Impulse, Impact, and Momentum
Momentum is the amount of motion a body possesses
Impulse is the application of an external force over a short time period
Impact is the application of an external force
Momentum is created by an impulse and is lost through impact
Impulse and impact are both associated with bodies that are changing
their state of motion by experiencing large accelerations over relative
short time periods
Collision or impact skills can sometimes manipulate the time of
contact and reduce the magnitude of the external force
To increase impulse, a sprinter must increase the net external force
per step
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Newton’s Third Law: Action-Reaction
Every action has an equal and opposite reaction
The two acting forces are equal in magnitude, but opposite
in direction
Example:
-the sprinter exerts a force
onto the blocks, and simultaneously
the blocks exert an equal force back
onto the sprinter
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Maximizing Height and Range
To maximize vertical distance (height), one must
maximize the takeoff velocity and take off vertically
To maximize horizontal distance (range), one must
maximize the takeoff velocity and take off at an angle of
45 degrees to the horizontal
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Taking off and Landing at Different
Heights
If the human body is the projectile,
a higher centre of mass can be achieved
by elongating the body at takeoff and by
raising as many body parts as possible
The range a projectile will travel
increases if the takeoff height is greater
than the landing height, and decrease if
the takeoff height is less than the
landing height
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Air Resistance
Another force acting on a projectile
Will change the state of motion of the
projectile and its path
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Effects of air resistance on a discus
Discus with direction of travel and relative
flow velocity vectors superimposed
Free body diagram of a discus indicating lift,
drag, and gravity vectors
Angle of attack (theta) defined for a discus
with direction of travel and longitudinal axis
drawn.
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Equilibrium
The state of a system that is not experiencing any change in its direction or
speed
Static equilibrium
System is at rest
Dynamic equilibrium
System is moving
with constant velocity
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Balance
Process whereby the body’s state of
equilibrium is controlled for a given
purpose
The base of support and location of the
centre of mass is manipulated
If the line of gravity passes through some
part of the body’s base of support, the
body will be balanced
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Stability
A measure of the difficulty with which equilibrium can be
disturbed
A net external force is required to overcome the static
equilibrium of a sprint start
There is a trade-off between maximizing stability and
acquiring speed off a mark
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To increase stability in static equilibrium
Increase the base of support
Increase the inertia of the body
Decrease the vertical distance between the centre of mass
and the base of support
Increase the distance between the point where a vertical
line from the centre of mass intersects the base of support
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Qualitative Analysis of Human Motion
The study of human motion, in the absence of measuring, by observing
a movement and applying biomechanical principles in assessing
performance
It is subjective and yet systematic evaluation of a movement or skill
It is based on a direct visual observation of a movement or its video
recording
It is dependent on the constraints or limitations of an event or
performer
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