Kinetics Lecture Notes PDF

KINETICS Ob jectives Define force Compare between internal force and external force Explain the characteristics of force Compare between force systems and give examples for each one Force - A force is a physical quantity that can accelerate, decelerate and or deform a body or maint...

KINETICS Ob jectives Define force Compare between internal force and external force Explain the characteristics of force Compare between force systems and give examples for each one Force - A force is a physical quantity that can accelerate, decelerate and or deform a body or maintain it in equilibrium. - It is a push or pull which does not necessarily produce motion. – It is that which produces, prevents, or modifies motion Classification of forces The force may be internal or external A- Internal forces; acting inside the body such as: 1- Muscular (M) and ligamentous forces. 2- Joint reaction force (J). 3- Intra-articular joint friction B- External forces; acting from outside the body such as: 1- Gravitational force (W). 2- Ground reaction force. 3- External resistance (R) Characteristics of Force Any force can be described according to four factors The characteristics of force are listed in Table (2) with reference to internal and external forces: 1- Magnitude: amount of force whether internal or external. 2- Point of application: point at which the force is applied. 3- Direction. ( oblique or horizontal or vertical ) As the force is a vector quantity it can be represented graphically by a vector which is a directed straight line (arrow). The length of the vector (arrow) is proportional to the magnitude of the force. The tail of the arrow represents the point of application and placed on the object at the point of application. The direction of the force is indicated by the arrow head. Table 2. Characteristics of force with reference to internal and external force. Force Characteristics Internal Force External force Magnitude Point of Application Direction  Force systems Force system = any group of two or more forces Two or more forces may be: Colinear: acting along the same action line Coplanar: acting in the same plane Concurrent: acting in the same point Types of force systems Linear force Parallel force Concurrent system system force system  Linear force system (colinear) - When all the forces occur along the same action line - Forces may act in the same direction or opposite direction - May produce tension or compression effects  Examples 1- Psoasmajor and iliacus muscles act along the same action line, point of application, and same direction. The resultant force equals the magnitude of the two forces. Weakness of one muscle will reduce the magnitude of the resultant force 2- Trapeziusmuscle on both sides act along the same action line, but in opposite directions. Equilibrium occurs when muscle forces are equal in both sides. Weakness on one side causes the resultant force to be bigger on the other side, resulting in lateral deviation of the spine (scoliosis).  Parallel force system Parallel force system: where two or more coplanar forces in the same or opposite direction are parallel in orientation. An example : Hamstring muscles components: medial (semitendinosus& semimembranosus) and lateral (biceps femoris) The medial and lateral forces act in the same plane to produce knee flexion  Force couple This is a special case of parallel force system are two coplanar forces equal in magnitude and opposite in directions. These two forces cause a turning effect. an example :. 1- upward rotation of scapula ( upper fibers of trapezius and serratus anterior ) Upper fibers of trapezius Serratus anterior  Concurrent force system Concurrent force system: These are forces originated in different direction. They meet at a common point or their extension meets at a point. This is the commonest type of force system especially in the human muscular system. Forces do not lie along the same line of action, but form an angle with each other Example: sternaland clavicularparts of the pectoralis major Deltoid muscle: Anterior fibers: flex the arm Posterior fibers: extend the arm  Clavicular head Sternal head Concurrent force system Example: Pectoralismajor