Post on 09-Apr-2018
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Force-types of contact forces
Forces exist in nature that affect the way
humans move.
A common classification is to describethese as contact and non-contact forces.
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Non-contact and contact forces
Non-contact
Gravity is the major external force actingon the body.
Contact
Reaction force
Muscle force
Elastic force
Friction
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Types of forces-contract and non-
contact Forces cause predictable and measurable
responses to the human body whenobjects interact with the human body.
These responses are: Resistive counter-forces
Deformations
breakage
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Contact forces
Reaction forces
Example- A runner experiences the
following ground reaction forcecomponents at one point in time during thestance phase.
Anterior-posterior (Fx) 250 N (positive=forward
Vertical (Fy) = 800 N (positive upward)
Medio-lateral (Fz) 60 N (positive lateral)
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An example of the vertical ground reaction
force from a force platform
Fy
Fz
Fx
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Vector components of ground
reaction force Check the components to draw:
X-component
Y-component Z-component
xyResultant of the horizontal components
xyzResultant of all three components
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Ground reaction force
FX-component of the ground reactionforce
FY-component of the ground reactionforce
FZ-component of the ground reactionforce
FxyHorizontal component of the groundreaction force
FxyzResultant ground reaction force
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Deformation-strain
There will be a change in shape or size, of a structure that is
composed of a deformable material. Strain is a measure of deformation and is an unit change in
the shape or size of the material.
There two types of strain:
Normal or longitudinal strain - measure of elongation or
contraction of material. Shear strain.- a measure of the relative rotation of the two
materials from their original perpendicular location.
Normal or longitudinal strain
Shear strain = change in angle between two elements
that were originally at right angles Normal strain = change in length
Original length
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Unloaded
TensionCompression bending
ShearTorsion
Combined
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Deformation
Mechanical properties provide a measure
of a materials ability to resist deformation
when subjected to externally applied
forces.
These properties can be determined
experimentally.
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Structural properties
Structural properties:
energy absorbed,
stiffness,
ultimate load and ultimate elongation. These structural properties are dependant on a
number of parameters,
the material properties of the tissue substance,
the geometry of the tissue (cross sectional area,length and shape)
and the properties of the bone-substance andmuscle-substance junctions.
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Stress Stress
An applied force is known as stress.
Normal stress = forceCross sectional area over which force acts
SI units are 1 Nm-2 = 1 Pascal
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Stress-strain curve plotted by converting measures of force and
deformation appropriately to represent material behavior.
2%
Strain offset Strain mm/mmElongation at failure
Yield point
True stress strain
Engineering stress strainfracture
StressN/mm
Ultimate strength
Yieldstrength
Elastic regionPlastic region
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Strain-stress properties of body
tissue Bone
Demonstrates a linearly elastic response from
the onset of loading. Substantial plastic
deformation occurs when bone is loaded ineither tension or compression. Strength and
elastic modulus of bone tissue are higher when
loaded at high strain rates, with less energy
absorbed than at lower strain rates.
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Articular cartilage
Articular cartilage exhibits viscoelastic
behavior in tension, appearing stiffer with
increasing strain. Cartilage behaves
elastically when subjected to sufficiently
fast load application.
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Ligaments
Ligaments pulled in tension demonstrates a
force elongation curve shown by small
nonlinear toe region, a relatively large linear
region and often a second nonlinear region thatmay plateau. The toe region corresponds to low
forces associated with everyday PA. Ligaments
strain-stress is associated with their structure
and material as they attach directly into bone.
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Tendons
Tendon similar force elongation curve as
ligaments shown by very small (0-3%) nonlinear
toe region (due to straightening of crimped
collagen fibrils), a relatively large linear regionup to 4% and often a second nonlinear region
that may fail up to 8-10% - elongate or break.
Tendons efficiently transmit force without
dissipating much energy during activity (90-96%).
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Question
Discuss the torsion stress on bone in two
cross sections of a tibia the distal and
proximal ends.
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Elastic force Elasticity:
When a force is applied to a material, the material undergoes a change
in its length, so F = k A measure of the ability to reform after being deformed.
Newtons Law of Coefficient of elasticity or restitution:
A degree of reformation or restitution of a deformed body that occursafter impact. The material when deformed stores energy known asstrain energy.
Velocities of two materials before u1 and after u2 Velocities of two materials after impact v1 and v2 Velocities after impact v1 -v2 = -e (u1-u2)
In the case of a rigid body i.e.the floor u2 and v2 are zero
@the coefficient of restitution e = - v1u1
or e =
v1
-v2u1-u2
It can also be written e = h rebound height
H start height
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Force applied to a spring Stiffness is defined as the amount of force
necessary to extend the body one unit of
length (N/
m).
Work done to stretch the spring 1 mm is
greater at longer lengths.
Length mm
Force N
5
10
15
0
2 4 6 8 10 12
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Force can be internal relative to a
system. Internal force Muscle force is a major internal force creating
movements of body segments.
Ligaments and tendons also apply forces to
create and restrict limb movements.
The body has the ability to also use viscoelastic
properties to create force. These forces are not easily measurable in the
human body.
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Bone
Mineral crystals within bone tissue
transmit large forces without significant
dissipation of energy; deformation is small
and primarily elastic. Bone gives structural
rigidity to the body.
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Tendon
Tendon (viscoelastic) composition and tensilestiffness reflect their role in generating motion byefficiently transmitting muscle contraction forces
across joints. Tendons are stronger enough to sustain high
tensile forces that result from muscle contractionduring joint motion, yet are flexible enough toangle around bone to change the final directionof muscle pull.
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Ligament and articular cartilage
Ligaments and articular cartilageexperience relatively large deformations,dissipating energy through viscoelastic
and poroelastic processes. The ligaments are pliant and flexible,
allowing movement of bones, but arestrong and inextensible offering suitableresistance to applied forces.
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Muscles
There are 430 muscles in the body
The most vigorous movements are
conducted by only 80 pairs. Muscles provide strength and protection to
the skeleton by distributing loads and
absorbing shock; they enable bones to
move at joints and provide body posture
against force.
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Muscles
Maximal tension is produced when the
muscle fibre is at resting length (slack).
If the muscle is shorter, tension falls ofgradually at first, then rapidly, and if the
muscle is longer than resting length
tension progressively decreases.
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Muscle force
Motive and resistive force
One body segment can exert a force on another,causing movement in that segment that is notdue to muscle action.
Joint forces account for inertial forces andgravity, they do not represent internal contactforces.
The bone on bone (contact) forces depend on
the level of muscle activity. The bone on bone and joint forces may act in
different directions.
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Joint reaction force of the knee with
its shear and compressive
components
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Muscle force vector, angle of pull
and its vertical and horizontal
components
Fx
Fy
Biceps brachii muscle force vector
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Net muscle force
Fmc
Fms
Fmc
Fms
0.55 rad
0.35 rad
a)
b)
Fm
Fmc Fms
c)
Geometric composition of the resultant muscle Fm
Due to activation of both clavicular Fmc and sternal
Fms components of the pectoralis major muscle.
a) orientation of the two vectors b) graphic addition
c) Calculation of the resultant vector.
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Friction force
Friction: Ffr= N
Fr = Friction force = = coefficient of friction and N =normal force
A force exerted between two contacting surfaces thatslide past each other.
Factors that effect friction:
Texture of surfaces
Force or pressure between the two surfaces (normal orperpendicular force)
Actual contact area
Other conditions wet or dry surface
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Quantifying friction force
A force acting at a point can be
resolved into a normal and tangential
components.
Normal force
Coefficient of friction
= Ffr/N
The direction of the kinetic frictional force
is opposite the direction of motion of the
object it acts on.
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Coefficient of friction
The coefficient of frictionis the ratio of the frictionforce to the normal force.
= Ffr Fn is the coefficient of
static friction, Ffr is themaximum static friction
force and Fn is theperpendicular forcepressing the two surfacestogether.
F fr Tangential force
F Fn
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Two types of coefficients of
friction
Coefficient of static friction amount of
force that is required before an object willbegin to move.
Coefficient of kinetic friction amount offorce required to keep an object moving.
Coefficient of kinetic friction< coefficient of static friction
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Rolling friction
Rolling friction designates theratio of the horizontal force
necessary to induce the wheel
rotation to the weight (verticalor normal force) that acts on
the wheel.
Discuss what you see.
R
F hoz
F verE
F ver
F hoz
E R
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Question
A 12 kg mass is pushed across a horizontalsurface by a force of 80 N inclined at an angle of30 with the horizontal. The coefficient of sliding
friction is 0.35.a. Make a free body diagram of the mass andfind the normal force acting on the mass.b. Find the force of friction acting on the mass.c. Find the acceleration of the mass as it moves
across the surface.