Kinetic Concepts for Analyzing Human Motion By Dr

Kinetic Concepts for Analyzing Human Motion By Dr. Saidatul Ardeena Bt Awang, Ph. D. Biomedical Electronics Engineering Universiti Malaysia Perlis Mc. Graw-Hill/Irwin © 2014 The Mc. Graw-Hill Companies, Inc. All rights reserved.

Basic Concepts Related to Kinetics What is mass? • Quantity of matter composing a body (dog, tree, desk, basketball, you) • Represented by m • Units are kg 3 -2

Basic Concepts Related to Kinetics § In international competition, the men's shot has a mass of 7. 26 kg (16 lb. ) and the women's shot has a mass of 4 kg (8. 8 lb. ). § The NBA regulation basketball has a mass of 0. 624 kg (1. 37 lb. ). (Mass is proportional to weight but note that mass is not related to size. ) 3 -3

Basic Concepts Related to Kinetics What is inertia? • Tendency to resist change in state of motion • Proportional to mass • Has no units! 3 -4

Basic Concepts Related to Kinetics Clearly, the weight bar will stay in place in the absence of being lifted because of its inertia. 3 -5

Basic Concepts Related to Kinetics What is force? • A push or a pull • Characterized by magnitude, direction, and point of application • F = ma • Unit is the Newton (N) 3 -6

Basic Concepts Related to Kinetics While there is no motion occurring, boy and dog are exerting equal and opposite forces on the rope. 3 -7

Basic Concepts Related to Kinetics What is a free body diagram? Force applied by racquet Air resistance Ball being struck by a racquet Ball weight (Diagram showing vector representations of all forces acting on a defined system) 3 -8

Basic Concepts Related to Kinetics What is a net force? • The single resultant force derived from the vector composition of all the acting forces • The force that determines the net effect of all acting forces on a body 3 -9

Basic Concepts Related to Kinetics What is a torque (T)? • The rotary effect of a force • The angular equivalent of force • Also known as moment of force 3 -10

Basic Concepts Related to Kinetics What is a torque? T = Fd d = 2 m axis F = 10 N T = (10 N)(2 m) 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) 3 -11

Basic Concepts Related to Kinetics What is the center of gravity? • Point around which a body’s weight is equally balanced in all directions • Point that serves as an index of total body motion • Point at which the weight vector acts • Same as the center of mass 3 -12

Basic Concepts Related to Kinetics 20 N 1 m 10 N CG 2 m The weights are balanced, creating equal torques on either side of the fulcrum. 3 -13

Basic Concepts Related to Kinetics If one child on a see-saw is heavier than the other, how can they balance? 3 -14

Basic Concepts Related to Kinetics What is weight? • Attractive force that the earth exerts on a body • wt. = mag (product of mass and the acceleration of gravity: -9. 81 m/s 2) 3 -15

Basic Concepts Related to Kinetics What is weight? • The point of application of the weight force is a body’s center of gravity • Since weight is a force, units of weight are units of force: N 3 -16

Problem 1: Solution: 3 -17

Problem 2: What is the mass of an object weighing 1200 N? Solution: 3 -18

Basic Concepts Related to Kinetics What is pressure? • Force per unit of area over which the force acts • Commonly used to describe force distribution within a fluid (e. g. blood pressure, water pressure) • Units are N/m 2 3 -19

3 -20 The surfaces of the vertebral bodies increase in surface area as more weight is supported.

Problem 4: Solution: 3 -21

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Basic Concepts Related to Kinetics 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/m 2 3 -23

Basic Concepts Related to Kinetics What is stress? 3 -24

3 -25 During the landing from a ski jump the axial impact force at the knee includes a component of anteriorly directed shear on the tibial plateau.

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Problem 3: How much compressive stress is present on the L 1, L 2 vertebral disc of a 625 N woman , given that approximately 45% of the body weight is supported by the disc: a) when the stands in anatomical position and b) when she stand erect holding a 222 N suitcase? (Assume that the disc is oriented horizontally and that its surface area is 20 cm^2) Solution: 3 -27

Basic Concepts Related to Kinetics Why would it hurt more to be stepped on by someone in high heels as compared to the same person wearing athletic shoes? 3 -28

Basic Concepts Related to Kinetics What is volume? • Space occupied by a body • Has three dimensions (width, height, and depth) • Units are m 3 and cm 3 3 -29

Basic Concepts Related to Kinetics What is density? • Mass per unit of volume • Represented with the small Greek letter rho: • Units are kg/m 3 3 -30

Basic Concepts Related to Kinetics What is specific weight? • Weight per unit of volume • Represented with the Greek letter gamma: • Units are N/m 3 3 -31

Basic Concepts Related to Kinetics A shot and softball have similar volumes, but the shot has greater mass and therefore greater density. Because weight is proportional to mass, the shot also has greater weight and therefore greater specific weight. 3 -32

Basic Concepts Related to Kinetics What is impulse? • The product of force and the time over which the force acts (Ft) • Units are Ns 3 -33

Basic Concepts Related to Kinetics What is compression? Original shape Compression (Pressing or squeezing force directed axially through a body) 3 -34

Basic Concepts Related to Kinetics What is tension? Original shape Tension (Pulling or stretching force directed axially through a body) 3 -35

Basic Concepts Related to Kinetics What is shear? Original shape Shear (Force directed parallel to a surface) 3 -36

Basic Concepts Related to Kinetics 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) 3 -37

Basic Concepts Related to Kinetics What is torsion? Neutral axis (Load producing twisting of a body around its longitudinal axis) 3 -38

Basic Concepts Related to Kinetics What is deformation? Load Yield Point Elastic Region Plastic Region Deformation (Change in shape) 3 -39 Ultimate Failure Point

Basic Concepts Related to Kinetics What are repetitive and acute loading? • Repetitive: repeated application of a subacute load that is usually of relatively low magnitude • Acute: application of a single force of sufficient magnitude to cause injury to a biological tissue 3 -40

Basic Concepts Related to Kinetics Load Magnitude Repetitive vs. acute loading Likelihood of Injury Frequency of Loading 3 -41

Vector Algebra What is vector composition? (Process of determining a single vector from two or more vectors by vector addition) 3 -42

Vector Algebra The composition of vectors with the same direction requires adding their magnitudes. 3 -43

Vector Algebra The composition of vectors with the opposite directions requires subtracting their magnitudes. 3 -44

Vector Algebra The tip-to-tail method of vector composition. 3 -45

Vector Algebra What is vector resolution? (Operation that replaces a single vector with two perpendicular vectors such that the vector composition of the two perpendicular vectors yields the original vector) 3 -46

Vector Algebra Vectors may be resolved into perpendicular components. The vector composition of each pair of components yields the original vector. 3 -47