PT-PAP 101-Gravity Lecture PDF

PT- PAP 101-Gravity September 17, 2024 www.gmu.ac.ae COLLEGE OF ALLIED HEALTH SEIENCES Objectives At the end of this lesson, the student should be able to Define Gravity Correlate gravity f...

PT- PAP 101-Gravity September 17, 2024 www.gmu.ac.ae COLLEGE OF ALLIED HEALTH SEIENCES Objectives At the end of this lesson, the student should be able to Define Gravity Correlate gravity forces on human movements Describe Segmental COG Understand what is Equilibrium Explain the Rules of Equilibrium  Introduction Gravity is an external force acting under normal circumstances which affects all the objects The first external force to be considered acting on human body Force of gravity is the pull of earth on a body or its segments and it’s a consistent one. Described as mutual attraction between the earth and an object Gravitational force is always directed towards the centre of earth which is towards the ground Gravity- Vector Gravity being a force is a vector quantity Point of application: COG Line of application: Centre of mass of object Direction : Towards centre of the earth Magnitude: Equal to Gravitational Force  Center of Gravity Although gravity acts at all COG is an hypothetical point points on an object or segment of at which all mass would an object, its application is given appear to be concentrated as Centre of Gravity (COG) and is the point at which the Center of Gravity is also referred force of gravity would as centre of Mass appear to act.  Symmetry and COG In a symmetrical object the In a asymmetrical object the COG will be located at the COG will be located geometric center of the towards the heavier end object where all the mass is evenly distributed around that point  COG in Human Body- s2 Segmental Whole When segments are combined, Each segment in body is gravity acting on combined segments acted on by the force of can be represented by a single COG gravity and has its own COG  COG in Human Body- s2 When the trunk is inclined forward, location of new COG lies outside the body Location of COG varies with each of the many and varied postures the body assumes  Line of Gravity The line of gravity is an imaginary vertical line from the centre of gravity to the ground or surface the object or person is on. It is the direction that gravity is acting upon the person or object. Gravity vector is commonly referred to as the Line of Gravity (LOG) LOG can best be visualized as a string with a weight on the end (a plumb line) with the string attached to the COG of an object Line of gravity  Positions and COG When the posture is good the LOG passes through the mid cervical and mid lumbar vertebrae and in front of thoracic vertebrae  Base of Support Base as applied to a rigid body is the area by which it is supported and in contact with the supporting surface In lying position posterior aspect of the whole body forms the base In stride standing BOS is an area as wide as the feet and as long as the distance between their outer borders Different types of walking aids are used to increase BOS of patients  Equilibrium Static equilibrium is a state where bodies are at rest; dynamic equilibrium is a state where bodies are moving at a constant velocity (rectilinear motion). In both cases the sum of the forces acting on them is zero. Equilibrium results when the forces acting upon a body are perfectly balanced.  Rule of Equilibrium The condition of equilibrium is most stable if, 1.The larger the BOS of an object, the greater the stability of that object 2.The closer the COG of the object is to the BOS, the more stable is the object. 3.An object cannot be stable unless its LOG falls within its BOS 4.Greater the mass of the object greater the stability 5.Greater the friction between the supporting surface and the BOS, the more stable the body will be.  Stability and COG For an object to be stable the LOG must fall with in the base of support (BOS) When LOG falls outside the BOS the object will fall(less stable) When BOS of an object is large, The LOG has more freedom to move with out passing beyond the limits of the base Examples………………………………… When a man stands with his legs spread apart,the base is larger side to side and the trunk can move a good deal in that plane with out displacing the LOG from COG When a person grasps or leans on another object that object can become part of BOS  When COG is low, movement of the object in space is less likely to cause the COG & LOG to fall outside the BOS The longer the LOG, the higher the COG, the less stable the object The shorter the LOG, the lower the COG, the more stable the object Relocation of Centre of Gravity Location of COG of a body depends not only on the arrangement of segments in space but also on the distribution of mass of the object Most common way to functionally redistribute mass in the body is to add external mass Every time we add an object to the body by wearing it,carrying it or using it the new COG for the combined body & the external mass will shift toward the additional weight;the shift will be proportional to the weight added 1. Results in shifting the COG down and to the right 2. Because his COG is now lower, he is theoretically more stable 3. Though he is standing only on one leg addition of crutches enlarges the base of support and thus improves the stability Holding a heavy suit case in right hand??? Results in shift of the COG up and to the right Because the LOG would move toward the right foot the man leans to the left to compensate Man leans laterally to the left not to relocate the COG but to bring the LOG back to the middle of base of support