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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.
3-6
Basic Concepts Related to Kinetics
What is inertia?
• tendency to resist change in state
of motion
• proportional to mass
• has no units!
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-7
Basic Concepts Related to Kinetics
Clearly, the weight bar will stay in place in the absence of being
lifted because of it’s inertia.
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-8
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)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-9
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)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-11
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
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-12
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
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-13
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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By Susan J. Hall, Ph.D.
3-14
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.
3-15
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.
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-20
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)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-21
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
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-22
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
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-23
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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By Susan J. Hall, Ph.D.
3-24
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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By Susan J. Hall, Ph.D.
3-26
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
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-29
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.
3-30
Basic Concepts Related to Kinetics
What is specific weight?
• weight per unit of volume
• represented with the Greek
letter gamma: 
• units are N/m3
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-31
Basic Concepts Related to Kinetics
What is impulse?
• the product of force and the time
over which the force acts
(Ft)
• units are Ns
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-32
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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By Susan J. Hall, Ph.D.
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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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By Susan J. Hall, Ph.D.
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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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By Susan J. Hall, Ph.D.
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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)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-37
Basic Concepts Related to Kinetics
What is torsion?
Neutral
axis
(load producing twisting of a body
around its longitudinal axis)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-38
Basic Concepts Related to Kinetics
What is deformation?
Load
Yield
Point
Elastic
Region
Ultimate
Failure
Point
Plastic
Region
Deformation
(change in shape)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
3-39
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
Basic Biomechanics, 6th edition
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
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
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Vector Algebra
What is vector composition?
(process of determining a single
vector from two or more vectors
by vector addition)
Basic Biomechanics, 6th edition
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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By Susan J. Hall, Ph.D.
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