WEB Force-3-Types of Contact Forces

8/8/2019 WEB Force-3-Types of Contact Forces

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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.

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WEB Force-3-Types of Contact Forces