2. TOPICS
• Definition
• Unit of force
• Classification
• External force
• Internal force
• Gravitational force
• Linear force system
• Few types of force
• Torque
3. FORCE
• Definition – a force (F) is described by the acceleration (a) of the
object to which the force is applied, with the acceleration being
directly proportional to the mass (m) of that object; that is,
Force = (mass)(acceleration)
F =(m)(a)
A force, simplistically speaking, is a push or a pull exerted
by one object or substance on another.
4. UNIT - THE UNIT FOR A FORCE (A PUSH OR A PULL)
IN THE SI SYSTEM IS THE NEWTON (N)
F
O
R
C
E
External Gravitational
Wind, Water
Internal Muscles,
Ligaments
5. EXTERNAL FORCE
• External force – A force from sources outside the body , which pushes or pulls
the body.
• Examples of external forces that may exert a push or pull on the human body
or its segments are wind (push of air on the body), water (push of water on
the body) , Gravity.
• Some forces, such as atmospheric pressure (the push of air pressure), work
both inside and outside the body, but — in our definition—are considered
external forces because the source is not a body structure.
6. GRAVITATIONAL FORCE
• Gravity (g), the attraction of the Earth’s mass to another mass, is an external
force that under normal conditions constantly affects all objects.
• Because gravity is the most consistent of the forces encountered by the body,
gravity should be the first force to be considered when the potential forces
acting on a body segment are identified.
7. INTERNAL FORCE
• Internal forces are forces that act on structures of the body and arise from
the body’s own structures (that is, the contact of two structures within the
body).
• A few common examples are the forces produced by the muscles (pull of the
biceps brachii on the radius), the ligaments (pull of a ligament on bone), and
the bones (the push of one bone on another bone at a joint).
8.
9. EFFECTS OF THESE TWO TYPES OF FORCES
• External forces can either facilitate or restrict movement.
• Internal forces are most readily recognized as essential for initiation of
movement. However, it should be apparent that internal forces also control or
counteract movement produced by external forces, as well as counteracting
other internal forces.
10. LINEAR FORCE SYSTEM
• A linear force system exists whenever two or more forces act on the same
segment, in the same plane, and in the same line (their action lines, if
extended, overlap).
• Forces in a linear force system are assigned positive or negative signs.
11. FEW TYPES OF FORCES
• DISTRACTION FORCE – A net force that moves a bony segment away from its
adjacent bony segment is known as a distraction force. A distraction force tends
to cause a separation between the bones that make up a joint.
• TENSILE FORCE – Two forces on an object which are opposite in direction
produces tension in that object which is known as tensile force.
• SHEAR FORCE – A shear force is any force (or the component of a force) that is
parallel to contacting surfaces (or tangential to curved surfaces) and has an
action line in the direction of attempted movement.
• FRICTION FORCE – Friction forces are always parallel to contacting surfaces (or
tangential to curved surfaces) and have a direction that is opposite to potential
movement.
• TORSIONAL FORCE – A torsion creates a “twist” within the structure of the
segment.
13. TORQUE
• Two forces that are equal in magnitude, opposite in direction, and applied to
the same object at different points are known as a force couple.
• A force couple will always produce pure rotatory motion of an object.
• The strength of rotation produced by a force couple is known as torque (T),
• Torque is a product of the magnitude of one of the forces and the shortest
distance (which always will be the perpendicular distance) between the
forces:
T = (F)(d)
