Transcript KIN 481

Chapter 3
Kinetic Concepts for
Analyzing Human
Motion
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
McGraw-Hill/Irwin
© 2012 The McGraw-Hill Companies, Inc. All rights reserved.
Laws of motion
•
Newton’s first law (law of inertia)
–
•
Body in motion tends to stay in motion at the same
speed in the same direction; body at rest tends to
stay at rest unless acted upon by an outside force.
What effect does gravity have on this?
Types of Motion
• Linear (translation)
• Curvilinear
• Angular (rotary)
Second law of motion
• Law of acceleration: change in acceleration of a body
occurs in the same direction as the force that caused
it, and is directly proportional to the force causing,
and inversely proportional to the mass of the body.
• Acceleration: change in rate of motion.
• Can be + or -.
• Mass: quantity of matter in a body
Third law of motion
• Law of reaction: for every action, there is an equal
and opposite reaction.
• This allows propulsion.
• Friction: force resulting from resistance between two
objects.
• Sometimes we want to increase this to enhance
performance; sometimes we want to decrease it.
Basic Concepts Related to Kinetics
What is mass?
• quantity of matter composing a body
(dog, tree, desk, swimming pool,
you)
• represented by m
• units are kg
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
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Basic Concepts Related to Kinetics
What is inertia?
• tendency to resist change in state
of motion
• proportional to mass
• has no units!
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By Susan J. Hall, Ph.D.
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Basic Concepts Related to Kinetics
Clearly, the weight bar will stay in place in the absence of being
lifted because of it’s inertia.
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By Susan J. Hall, Ph.D.
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Basic Concepts Related to Kinetics
What is force?
• a push or a pull
• characterized by magnitude,
direction, and point of
application
• F = ma
• unit is the Newton (N)
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By Susan J. Hall, Ph.D.
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Force
• Force: a push or pull in a direction.
• F=mxa
• Kinetic energy:1/2 mass x velocity²
Basic Concepts Related to Kinetics
What is a free body diagram?
Force applied
Ball being
struck by a
racquet
Air resistance
by racquet
ball weight
(diagram showing vector representations
of all forces acting on a defined system)
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By Susan J. Hall, Ph.D.
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Basic Concepts Related to Kinetics
What is a net force?
• the single resultant force derived
from the vector composition of
all the acting forces
• the force that determines the net
effect of all acting forces on a
body
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By Susan J. Hall, Ph.D.
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Basic Concepts Related to Kinetics
What is a torque (T)?
• the rotary effect of a force
• the angular equivalent of force
• also known as moment of force
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By Susan J. Hall, Ph.D.
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Basic Concepts Related to Kinetics
What is a torque?
T = Fd
F = 10N
T = (10N)(2m)
T = 20 Nm
d = 2m
axis
T = Fd (the product of force and the
perpendicular distance from the force’s
line of action to the axis of rotation)
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Basic Concepts Related to Kinetics
What is the center of gravity?
• point around which a body’s weight is
equally balanced in all directions
• point that serves as an index of total
body motion
• point at which the weight vector acts
• same as the center of mass
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By Susan J. Hall, Ph.D.
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Center of Gravity
• Point at which all body’s mass and weight is
evenly distributed in all directions.
• Center of gravity is dependent upon:
– Body shape (anthropometrics, gender)
– Body position
• Base of support
Principles of balance and stability
A person has balance …
• When the c/g falls
within the base
• In direct proportion to
size of the base
• Depending on weight
(mass).
• Depending on height of
c/g
• Depending on how
close c/g is to edge of
base.
• When c/g is nearer the
center of the base.
Principles of balance and stability
• To meet an oncoming force, stability is
increased by:
– moving c/g toward the force.
– Enlarging base on the side of the force
• Equilibrium enhanced by increasing friction
between body and surface it contacts.
More principles of balance and stability
• Rotation about an axis increases stability.
• Locomotion (walking) is created by dislocating
the c/g from the base and then reestablishing
it within the base.
• Movement in a direction may be enhanced by
moving the c/g within the base in that
direction.
Basic Concepts Related to Kinetics
10N
20N
CG
1m
2m
The weights are balanced, creating equal
torques on either side of the fulcrum.
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By Susan J. Hall, Ph.D.
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Basic Concepts Related to Kinetics
What is weight?
• attractive force that the earth exerts
on a body
• wt. = mag (product of mass and the
acceleration of gravity: -9.81 m/s2)
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By Susan J. Hall, Ph.D.
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Basic Concepts Related to Kinetics
What is weight?
• the point of application of the weight
force is a body’s center of gravity
• since weight is a force, units of
weight are units of force: N
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Basic Concepts Related to Kinetics
What is pressure?
• force per unit of area over which the
force acts
• commonly used to describe force
distribution within a fluid (e.g.
blood pressure, water pressure)
• units are N/m2
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Basic Concepts Related to Kinetics
What is stress?
• force per unit of area over which
the force acts
• commonly used to describe force
distribution within a solid
• units are N/m2
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Pressure and Stress
• Pressure = force ÷ area
• The greater the area over which a force is
distributed, the lower the pressure (amount of
stress).
• Muscles and bones adapt to stress by
hypertrophy.
Basic Concepts Related to Kinetics
What is stress?
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Bone adaptations to stress
• Hypertrophy: bones increase in density when
subjected to continuous stress.
• Bone atrophy and osteoporosis
• Bony insertion points for tendon attachments
(muscles) also increase in size to
accommodate for increased tensile stress.
Tendon and ligament responses to
stress
• Tendons and ligaments increase in size and
strength to accommodate for increased tensile
stress.
Basic Concepts Related to Kinetics
What is volume?
• space occupied by a body
• has three dimensions (width, height,
and depth)
• units are m3 and cm3
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Basic Concepts Related to Kinetics
What is density?
• mass per unit of volume
• represented with the small Greek
letter rho: 
• units are kg/m3
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By Susan J. Hall, Ph.D.
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Basic Concepts Related to Kinetics
What is specific weight?
• weight per unit of volume
• represented with the Greek
letter gamma: 
• units are N/m3
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Basic Concepts Related to Kinetics
What is impulse?
• the product of force and the time
over which the force acts
(Ft)
• units are Ns
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Mechanical loading
• Types of forces:
– Tension
• Muscles pulling on bone.
– Compression
• Gravity compressing spine or long bones.
– Shear
• Getting hit in the side of the knee.
– Bending
• Greenstick fractures
– Torsion
• Twisting knee
Basic Concepts Related to Kinetics
What is compression?
Original
Shape
Compression
(pressing or squeezing force directed
axially through a body)
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Basic Concepts Related to Kinetics
What is tension?
Original
Shape
Tension
(pulling or stretching force directed
axially through a body)
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Basic Concepts Related to Kinetics
What is shear?
Original
Shape
Shear
(force directed parallel to a surface)
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Basic Concepts Related to Kinetics
What is bending?
Compression
Tension
(asymmetric loading that produces tension
on one side of a body’s longitudinal axis
and compression on the other side)
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By Susan J. Hall, Ph.D.
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Basic Concepts Related to Kinetics
What is torsion?
Neutral
axis
(load producing twisting of a body
around its longitudinal axis)
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Basic Concepts Related to Kinetics
What is deformation?
Load
Yield
Point
Elastic
Region
Ultimate
Failure
Point
Plastic
Region
Deformation
(change in shape)
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By Susan J. Hall, Ph.D.
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Basic Concepts Related to Kinetics
What are repetitive and acute loading?
• repetitive: repeated application of a
subacute load that is usually of
relatively low magnitude
• acute: application of a single force of
sufficient magnitude to cause
injury to a biological tissue
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By Susan J. Hall, Ph.D.
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Basic Concepts Related to Kinetics
Load Magnitude
Repetitive vs. acute loading
Likelihood of Injury
Frequency of Loading
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Vector Algebra
What is vector composition?
(process of determining a single
vector from two or more vectors
by vector addition)
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By Susan J. Hall, Ph.D.
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Vector Algebra
Vectors may be resolved into perpendicular
components. The vector composition of
each pair of components yields the original
vector.
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