# Force-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 describe
these as contact and non-contact forces.
Non-contact and contact forces
• Non-contact
• Gravity is the major external force acting
on the body.
• Contact
• Reaction force
• Muscle force
• Elastic force
• Friction
Types of forces-contract and non-
contact
• Forces cause predictable and measurable
responses to the human body when
objects interact with the human body.
These responses are:
• Resistive counter-forces
• Deformations
• breakage
Non-contact-Gravitational force
• F= Gm1m2
r2
• G = universal gravitational constant
• M1 = mass of one object
• M2 = mass of the other subject
• r = distance between mass centres of the
objects
Contact forces

• Reaction forces
• Example- A runner experiences the
following ground reaction force
components at one point in time during the
stance phase.
• Anterior-posterior (Fx) – 250 N (positive
=forward
• Vertical (Fy) = 800 N (positive upward)
• Medio-lateral (Fz) 60 N (positive lateral)
An example of the vertical ground reaction
force from a force platform

Fz
Fx

Fy
Vector components of ground
reaction force
•   Check the components to draw:
•   X-component
•   Y-component
•   Z-component
•   xy Resultant of the horizontal components
•   xyz Resultant of all three components
Ground reaction force
• FX-component of the ground reaction
force
• FY-component of the ground reaction
force
• FZ-component of the ground reaction
force
• Fxy Horizontal component of the ground
reaction force
• Fxyz Resultant ground reaction force
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
Tension                   bending
Compression

Shear         Torsion

Combined
Deformation
• Mechanical properties provide a measure
of a material’s ability to resist deformation
when subjected to externally applied
forces.
• These properties can be determined
experimentally.
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 and
muscle-substance junctions.
Stress
• Stress
• An applied force is known as stress.
• Normal stress = force
Cross sectional area over which force acts
• SI units are 1 Nm-2 = 1 Pascal
Stress-strain curve plotted by converting measures of force and
deformation appropriately to represent material behavior.

Stress N/mm                               True stress strain
Ultimate strength
Engineering stress strain
Yield    Yield point
strength                                       fracture

Elastic region
Plastic region

2%                    Elongation at failure
Strain offset Strain mm/mm
Strain-stress properties of body
tissue
• Bone
• Demonstrates a linearly elastic response from
deformation occurs when bone is loaded in
either 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.
Articular cartilage
• Articular cartilage exhibits viscoelastic
behavior in tension, appearing stiffer with
increasing strain. Cartilage behaves
elastically when subjected to sufficiently
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 that
may 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.
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 region
up 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%).
Question
• Discuss the torsion stress on bone in two
cross sections of a tibia –the distal and
proximal ends.
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.
•    Newton’s Law of Coefficient of elasticity or restitution:
•    “A degree of reformation or restitution of a deformed body that occurs
after impact. The material when deformed stores energy known as
strain 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 = - v1
u1
or     e = v1 -v2
u1-u2
It can also be written e =  h rebound height
H start height
Force applied to a spring
• Stiffness is defined as the amount of force
necessary to extend the body one unit of
length (N/m).
15

Force N   10
5
0
2 4 6 8 10 12
Length mm
• Work done to stretch the spring 1 mm is
greater at longer lengths.
Force can be internal relative to a
• Internal force
system.
• 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.
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.
Tendon
• Tendon (viscoelastic) composition and tensile
stiffness reflect their role in generating motion by
efficiently transmitting muscle contraction forces
across joints.
• Tendons are stronger enough to sustain high
tensile forces that result from muscle contraction
during joint motion, yet are flexible enough to
angle around bone to change the final direction
of muscle pull.
Ligament and articular cartilage
• Ligaments and articular cartilage
experience relatively large deformations,
dissipating energy through viscoelastic
and poroelastic processes.
• The ligaments are pliant and flexible,
allowing movement of bones, but are
strong and inextensible offering suitable
resistance to applied forces.
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.
Muscles
• Maximal tension is produced when the
muscle fibre is at resting length (slack).
• If the muscle is shorter, tension falls of
gradually at first, then rapidly, and if the
muscle is longer than resting length
tension progressively decreases.
Muscle force
Motive and resistive force
• One body segment can exert a force on another,
causing movement in that segment that is not
due to muscle action.
• Joint forces account for inertial forces and
gravity, they do not represent internal contact
forces.
• 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.
Joint reaction force of the knee with
its shear and compressive
components
Muscle force vector, angle of pull
and its vertical and horizontal
components

Fy
θ

Fx

Biceps brachii muscle force vector
Net muscle force
b)                  c)
a)                   Fmc                           Fms
Fmc

Fms             Fms

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.
Friction force
• Friction: Ffr = μ N
• Fr = Friction force = μ = coefficient of friction and N =
normal force

• A force exerted between two contacting surfaces that
slide past each other.
• Factors that effect friction:

• Texture of surfaces
• Force or pressure between the two surfaces (normal or
perpendicular force)
• Actual contact area
• Other conditions – wet or dry surface
Quantifying friction force
• A force acting at a point can be
resolved into a normal and tangential
components.
• Normal force
• Coefficient of friction
• μ = Ffr/N
• The direction of the kinetic frictional force
is opposite the direction of motion of the
object it acts on.
Coefficient of friction
• The coefficient of friction
is the ratio of the friction
force to the normal force.

• μ = Ffr
•        Fn                          F fr Tangential force
• μ is the coefficient of
static friction, Ffr is the    F                      Fn
maximum static friction                     θ
force and Fn is the
perpendicular force
pressing the two surfaces
together.
Two types of coefficients of
friction
• Coefficient of static friction – amount of
force that is required before an object will
begin to move.
• Coefficient of kinetic friction – amount of
force required to keep an object moving.
•       Coefficient of kinetic friction
< coefficient of static friction
Rolling friction
• Rolling friction designates the
ratio of the horizontal force
necessary to induce the wheel
rotation to the weight (vertical
or normal force) that acts on
the wheel. F hoz
F hoz
• Discuss – what you see.
E
F ver   E    R         F ver        R
Question
• A 12 kg mass is pushed across a horizontal
surface by a force of 80 N inclined at an angle of
30° with the horizontal. The coefficient of sliding
friction is 0.35.
a. Make a free body diagram of the mass and
find 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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 views: 146 posted: 4/6/2010 language: English pages: 37