Transcript Basic Biomechanics, (5th edition) by Susan J. Hall, Ph.D.

Bellringer
Compare and explain in complete
sentences and formulas
how the third Newton’s law
is applied to find the resultant force.
© 2007 McGraw-Hill Higher Education. All rights reserved.
HOMEWORK
FINISH 50 % OF THE PROJECT
© 2007 McGraw-Hill Higher Education. All rights reserved.
Kinetic Concepts for
Analyzing Human
Motion
Identify and describe the different types of mechanical loads
that act on the human body.
Explain the effects of loading on human body.
Identify and describe the uses of available instrumentation for
measuring kinetic quantities.
What are : Mass1, inertia2, force3?
1. Quantity of matter composing a
body
2. Tendency to resist change in state
of motion, proportional to mass, has
no units!
3. A push or a pull acting on a body
and characterized by magnitude,
direction, and point of application.
Classification of forces

External and Internal Forces
 Internal forces : Forces acting within object (system) being
considered.
 External forces (contact force or distant force): Forces acting
on an object (system) as a result of its interaction with the
environment.

Contact force - Normal and Tangential Forces
 If a force acting on a surface is applied in a direction
perpendicular (normal) to that surface, then the force is
called a normal force.
 A tangential force is that applied on a surface in the direction
parallel to the surface.
Classification of forces

Tensile and Compressive Forces
A tensile force applied to a body will tend
to stretch or elongate the body (a).
A compressive force will tend to shrink
the body in the direction that it is applied
(b).
Classification of forces
Coplanar Forces
A system of forces is said to be
coplanar if all the forces are
acting on a two-dimensional
(plane) surface
Collinear Forces
A system of forces is collinear if all
the forces have a common line of
action.
To determine the magnitude of the
resultant force vector for a
collinear force system, all we have
to do is arithmetically add and/or
subtract the magnitude of forces
forming the force system.
Classification of forces
Concurrent Forces
A system of forces is
concurrent if the lines of action
of the forces have a common
point of intersection.


Figure (traction device)
Resolve the forces into their
components along the
horizontal and vertical
directions and subsequently
determine the force resultant,
and undertake an in-depth
analysis of the entire system.
Classification of forces
Parallel Forces
A set of forces form a parallel
force system if the lines of
action of the forces are parallel
to each other.
The forces on the forearm are:




the weight of the object,
the weight of the arm itself,
the tensions in the biceps muscle,
and the joint reaction force at the
elbow.
HUMAN MOVEMENT
During motion, every contact of foot with
the floor or ground generates an upward
reaction force.
F=mg - the vector that represents the
force of gravity acting on the object.
F=ma - the vector that represents the
instantaneous inertial force acting on
the object. In this diagram, inertial
forces accelerate the body toward
downward and to the right.
Vector (Fr) - the resultant or sum of
the gravitational and inertial forces
http://moon.ouhsc.edu/dthompso/gait/kinetics/GRFBKGND.HTM
HUMAN MOVEMENT


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
Vector Fr represents a force that is opposed
by a ground reaction force of equal
magnitude.
Because the ground reaction force is equal
and opposite, its vector's line of application is
the same as that of Fr, and it has the same
effect on the body and its joints.
The GRFV combines both gravity's effect on
the body and the effects of the body's
movement and acceleration (change of
velocity) in three planes of reference.
This makes the GRFV especially suitable for
the study of gait, during which the body's
various masses undergo complex
accelerations.
http://moon.ouhsc.edu/dthompso/gait/kinetics/GRFBKGND.HTM
What is a free body diagram?
Ball being
struck by a
racquet
Air
resistance
Force
applied by
racquet
ball
weight
A diagram showing vector representations
of all forces acting on a defined system.
What is A Free Body Diagram
• Free-body diagram are constructed to help
identify the forces and moments acting on
individual parts of a system and to ensure the
correct use of the equations of statics. In a freebody diagram, all known and unknown forces
and moments are shown.
• A force is unknown if its magnitude or direction is
not known. For the known forces, the correct
direction is indicated.
• If the direction of a force is not known, a
direction is chose for it. If this initial guess is not
correct, it will appear in solutions as negative
force.
What is the FBD of the following
activities?
Consider a person trying to
push a block to the right on a
rough surface as illustrated in
figure
There are three parts
constituting this system:
•The person
•The block
•The horizontal surface
representing the earth











Figure 5.4 shows the free-body diagram
of the system.
F = magnitude of the horizontal force F
applied by the person on the block.
F also is the magnitude of the force
applied by the block on the person
(Newton’s third law)
W1 = weight of the block
W2 = weights of the person
N1 = magnitude of force on the
horizontal surface applied by the
block
N2 = magnitude of force on the
horizontal surface applied by the
person
f1 and f2 are the magnitudes of
frictional forces f1 and f2
between the block, the person and
the horizontal surface.

Newton’s First Law
• States that a body that is originally at rest
will remain at rest, or a body in motion will
move in a straight line with constant velocity,
if the net force acting upon it is zero. i.e F=0
Forces are balanced
Objects at Rest
(v = 0 m/s)
a = 0 m/s2
Stay at rest
Objects in Motion
(v  0 m/s)
a = 0 m/s2
Stay in motion
(same speed
and direction)

Newton’s Second Law
• States that a body with a net force acting on it
will accelerate in the direction of that force, and
that the magnitude of the acceleration will be
proportional to the magnitude of the net force.
• In terms of an equation, the net force is equated
to the product of the mass times the
acceleration
Fnet = m*a
Forces are unbalanced
There is an acceleration
The acceleration depends
directly upon the “net force”
mass
The acceleration depends
inversely upon the object’s

Newton’s Third Law
• States that to every action there is
always an equal reaction, and that the
forces of action and reaction between
interacting bodies are equal in
magnitude, opposite in direction, and
have the same line of action.
What is a net force?
Vector sum of all acting forces
When all forces are balance, cancel
out each other – net force – 0.
Newton 1st law.
When a net force is present – Newton
2nd law.
ANGULAR KINETICS
Concentric And Eccentric Forces
Eccentric forces – does not pass
through the centre of the body, body
experience translation and rotation at
the same time.
Coupling Effect – 2 eccentric forces
of equal magnitude act at both sides
of a stick, body rotate without any
translation.
ANGULAR KINETICS
What is a torque?
The rotary effect on a body as a
result of an eccentric force or a
couple.
What is a torque?
axis
d = 2m
F = 10N
T = Fd
T = (10N)(2m)
T = 20 Nm
T = Fd (the product of force and the
perpendicular distance from the
force’s line of action to the axis of
rotation)
Also known as moment of force. Any
different between moment and torque?
Between torque and moment?
Open the door ? Torque.
Bend the board with fix end using body
weight? Moment.
Torque associated with rotational and
twisting action of applied force,
whereas Moment is related to the
force bending effect.
However, their mathematical
definition is the same. Therefore it is
sufficient to use moment to describe
both parameters.
What
is moment of Inertia?
A quantity that describes a body’s reluctance to
change its state of angular motion. The moment of
inertia must be specified with respect to a chosen
axis of rotation.
For a point mass the moment of inertia is just the
mass times the square of perpendicular distance
to the rotation axis, I = mr2.
It is analog of mass for linear motion.
M=Iα
M: moment;
I : moment of inertia
(rotational inertia)
a : angular acceleration
F=ma
F : Force
m: mass (linear inertia)
a : linear acceleration.
What is the center of mass?
x1
n
M   mi
i 1
mi  di Vi
d: density; V: volume
n
M  d  Vi
i 1
n
Mx   m i x i
i 1
Mass is
concentrated on
particular point
1 n
x   mi x i
M i 1
Sit up, torque and centre of mass
Extending the arms and
keep them behind the
head. The trunk centre
of mass is far from the
hip.
Extending the arms and
keep them by the side
of the body. The trunk
centre of mass is
closer to the head.
Sufficient
muscular
contraction to
produce muscle
torque to
overcome
resistive torque
due to trunk
weight
What is impulse?
The resulting motion is not only
depend on force (magnitude,
direction) also the duration of force
application.
Impulse = Ft (Ns)
A large impulse may resulted from a
small force acting for a relatively long
time or from large force acting for a
relatively short time.
Momentum
A vector, mass (m) multiplied by velocity (v), p =mv.
Newton 1st law state that p remain constant ( in the
absence of external forces), conservation of momentum.
Newton 2nd law can be express in terms of momentum as
well, provided the mass is constant.
ma = mdv/dt = d/dt (mv) = dp/dt = F; which can be
expressed as the rate of change of the momentum of a
body equals the net external force acting on it.
 MECHANICS
OF
DEFORMABLE BODY
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. Related to
deformable body.
Between Pressure and Stress?

Stress is an internal resistance to external load
where as pressure is external force applied upon
the surface.

We know that every materials is elastic in nature ,
so when a force is applied it tends to deform and
due to this deformation , there is an internal
resistance produced which tends to spring back
the molecules to their original position and this
internal resistance per unit area is know as Stress.
Pressure is nothing but force acting on a unit
area.

What is compression?
Original
Shape
Compression
(pressing or squeezing force directed
axially through a body)
What is tension?
Original
Shape
Tension
(pulling or stretching force directed
axially through a body)
What is shear?
Original
Shape
Shear
(force directed parallel to a surface)
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)
What is torsion?
Load producing twisting of a body
around its longitudinal axis.
Neutral
axis
Torsion?
Torsional fractures of Tibia are
not uncommon in football
injuries and skiing accidents –
foot fix, body twist.
Combined Loading
The presence of more than one
form of loading – most common
type of loading on the body.
THE EFFECTS OF LOADING ON HUMAN BODY
Force applied – acceleration and deformation.
The amount of deformation depends on the stiffness
of the objects. What is stiffness? Steeper
slope – high stifness.
Yield
Point
Ultimate
Failure
Point
Load
Small load - deform,
elastic - get back to its
original shape.
If exceed elastic limit,
deformation is
permanent.
Exceed ultimate failure
point – fracture.
Elastic
Region
Plastic
Region
Deformation
INJURIES
Several factors influencing injuries in
human:
Magnitude and direction of forces
The area over the force is distributed.
The material properties of the loaded
body tissues.
Elastic versus viscoelastic
behaviors
• For a constant applied strain
• An elastic material has a unique material
response
• A viscoelastic material has infinite material
responses depending on the strain-rate
VISCOELASTICS


Definition: time-dependent material behavior
where the stress response of that material
depends on both the strain applied and the
strain rate at which it was applied!
Examples
• biological materials
• polymer plastics
• metals at high temperatures
What are acute loading?



When a single force of sufficient
magnitude to cause injury acts
on a biological tissue, the injury
is term acute.
Microtrauma: causative force
Fall, rugby tackle, automobile
accident
What are repetitive loading?
A repeated application of relatively
small forces, the injury is term as
chronic injury or a stress injury.
Macrotrauma: causative force
THE AVAILABLE INSTRUMENTATION FOR MEASURING
KINETIC QUANTITIES


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Electromyography
Use to study neuromuscular functions
Two concepts:
• Muscle developed tension when electrically
stimulated
• Muscle produce voltage when developing
tension.
THE AVAILABLE INSTRUMENTATION FOR
MEASURING KINETIC QUANTITIES

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
Dynamography
Objectives:
Measurement of forces and pressure on
the plantar surface of the foot.
Provide graphical time history of
recorded force.
PRESENTATION FOR NEXT TUTORIAL
In a group of five write a 5 minuts
presentations of currently available
equipments for measuring kinetics
quantity.
Also write the task of each group members
to do the findings and presentation.
Please include the following items:
•
•
•
•
Name, types and classification (if available)
Each major component in the equipment.
How it is used? Purpose or objectives?
Advantages and disadvantageous of the
system describe