KMCH Basic Biomechanics.ppt

Basic Biomechanics
Part -I
Shashidhar Rao C
Prof.
KMCH College of Occupational Therapy

• We shall now consider how different
forces, both internal and external, may
affect range and quality of movement

Mass
Definition:
The mass of a body is the quantity of matter it
contains and the SI unit of mass is the
kilogram (Kg)
The density of the mass determines its size or
volume and this will remain constant.

Weight
Definition:
The weight of a body is the force it exerts on a
surface that supports it and the SI unit of
weight is the Kgf.
The weight of an object is produced by the
effect of the force of gravity pulling towards
the earth’s surface.

FORCE
Definition:
A force denotes either a push (compression)
or pull.
It either produces movement and or some
internal deformation of the object.
The SI unit of force is the Newton (N)

Newton
Definition: A Newton is the force which
produces an acceleration of one metre per
second per second [1m/s/s] when it acts on a
mass of 1 kg.
1kg=9.81N

Objects in motion or stationary are governed
by certain laws described as
NEWTON’s LAWS OF MOTION
FIRST LAW:
It states that an object will remain in a
constant state unless it is acted upon by an
applied force.
This tendency is called as INERTIA.

Second Law
Law of acceleration
It states that when a force is applied and an
object does change its ‘state’, the amount of
change of its velocity is dependent upon the
force applied, the direction of the force and
the mass of the object.

Third law
It states that for every action there is an equal
and opposite reaction.

Gravity
It is an invisible force that attracts or pulls all
objects to the centre of the earth.
The gravitational force exerted by the earth’s
gravitational field causes all objects to fall to
the ground at a speed of 9.81 metres per
second per second [9.81m/s/s]

Centre of gravity
Definition:
It is an imaginary point in the centre of an
object’s mass.
It is also referred to as center of mass.
c.g. of the body?

The ‘normal’ or average c.m. for individual
body segments have been calculated and
they are usually said to be at the junction
between the proximal 2/5th (40%) and the
distal 3/5th [60%) of the limb or part of a limb.

The LINE OF GRAVITY
Definition: Is an imaginary line dropped
vertically from the centre of mass of an object
to its supporting surface (BASE OF
SUPPORT).
BOS: Is the contact area of the supporting
surfaces plus any area that lies between these
points of contact.

Exercise
Observe for the change in the centre of Mass
OR Line of gravity when carrying a bucket of
water or a luggage bag
The line of gravity is continuously changing as
we perform every day activities.

Compare the BOS of the human body when
standing and as in Quadruped.
How can the BOS can be changed?...
What is the advantage of changing the BOS?
…
…
… Stability.

An object is most stable when its centre of mass is
low and the line of gravity falls at the centre of its
base of support.
An object is most stable with:
1. A large base of support
2. A low centre of mass/gravity
3. A line of gravity which falls in the middle of the
BOS
4. A symmetrical shape.

…accordingly there are three types of
equilibrium:
1. Stable equilibrium
2. Unstable equilibrium
3. Neutral equilibrium

Stable Equilibrium
When the centre of mass is low and the line of
gravity falls at the centre of its base of support
Unstable Equilibrium
An object on a very small base with a high centre of
mass and the line of gravity easily falls outside th
ebase of support

Neutral Equilibrium
If the object can be moved and yet the line of
gravity still falls within the base.
for example a rolling ball

FORCE
What is the definition of FORCE?
What produces force in the HUMAN BODY?
To describe a force it is necessary to know its:
1. Magnitude
2. Action line
3. Direction
4. Point of application

Magnitude
This is the amount of force applied, in
kilograms.
This may be from the:
-Weight of the limb or moving body part
-Additional resistance

Direction
This is described generally in terms such as
“vertical”, “Horizontal”, or there may be more
specific parameters such as “45 degrees
south” etc.
The direction of the force is indicated by the
direction of an action line with an arrow at the
tip.

Force is a vector quantity…i.e., it has both
direction and magnitude
A vector is represented by an arrow.
The length of the arrow is proportional to the
magnitude of the force;
The arrow head indicates the direction of the
force.

FORCE SYSTEMS
Any group of two or more forces is known as a
force system.
Two or more forces may be:
1) Colinear
2) Coplanar
3) Concurrent

Linear Force System
When all the forces occur along the same
action line.
When acting in the same direction, colinear
forces produce pushing or pulling.
When in opposite direction, such forces may
produce tension or compression

Coplanar (Parallel Force System)
When all the forces are coplanar and parallel to each
other but do not share the same action line.
These forces tend to produce rotatory effects
A special type of parallel force system is called a
force couple. In this case the parallel forces are
equal in magnitude but opposite in direction.

Concurrent Force System
When all the forces meet at the same point.
General Force System
When all the forces acting are in the same
plane but cannot be covered by the above
categories

Composition of Forces
It helps us to know the final effect of all these
forces, or the resultant, which is the simplest
force that can produce the same effect as all
the forces acting together.

This process of finding the resultant is called
the composition of forces. This can be
expressed as:
R = F1 + F2 + F3…Fn
= ∑F
Where R means the resultant
F means a force

• When the sum of all the forces acting on
the body equals zero, the force system is
said to be….
in equilibrium.
i.e. all the forces balance each other.
When the sum of all the forces does not
equal zero,…
motion occurs

• First law of Equilibrium:
• ∑F = 0

• When analyzing force systems, two basic
methods are used.
• 1. Algebraic method
• 2. Graphic method
• E.g. Tug of War…
• In the same direction
• In the opposite direction

• In graphic method the resultant is found by
drawing a line from the tail of the first vector
to the head of the final vector

• Force analysis of concurrent force system
• Graphic method:
• 1. Parallelogram method: it is used when
only two forces are applied to the same
point simultaneously.
• E.g. deltoid muscle activity

• 2. Triangle method:
• This is also used when two forces are
involved.
• However, one vector is placed from the tip
of the previous vector in the original
direction. The resultant vector R is found by
joining the tail of the first vector to the head
of the last

• 3. Trignometric method:
• This can be used if the value of the angle
between the vectors of forces is known.
• Then the resultant is found using:
• R= Sqrt. P2 + A2 – 2PA Cosθ
• Cos θ = Side adjacent / Hypotenuse

• A special situation occurs if the angle
between two concurrent forces is 90
degrees…
• …Pythagoras theorem is used to find the
resultant

• Polygon Method…
• This is used when more than two forces are
acting on a body at different angles and
different directions.
• Example: Russel’s Traction

KMCH Basic Biomechanics.ppt

More Related Content

What's hot (20)

Similar to KMCH Basic Biomechanics.ppt (20)

Recently uploaded (20)

KMCH Basic Biomechanics.ppt