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Chapter 3 Kinetic Concepts for Analyzing Human Motion Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. McGraw-Hill/Irwin © 2012 The McGraw-Hill Companies, Inc. All rights reserved. Laws of motion • Newton’s first law (law of inertia) – • Body in motion tends to stay in motion at the same speed in the same direction; body at rest tends to stay at rest unless acted upon by an outside force. What effect does gravity have on this? Types of Motion • Linear (translation) • Curvilinear • Angular (rotary) Second law of motion • Law of acceleration: change in acceleration of a body occurs in the same direction as the force that caused it, and is directly proportional to the force causing, and inversely proportional to the mass of the body. • Acceleration: change in rate of motion. • Can be + or -. • Mass: quantity of matter in a body Third law of motion • Law of reaction: for every action, there is an equal and opposite reaction. • This allows propulsion. • Friction: force resulting from resistance between two objects. • Sometimes we want to increase this to enhance performance; sometimes we want to decrease it. Basic Concepts Related to Kinetics What is mass? • quantity of matter composing a body (dog, tree, desk, swimming pool, you) • represented by m • units are kg Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-6 Basic Concepts Related to Kinetics What is inertia? • tendency to resist change in state of motion • proportional to mass • has no units! Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-7 Basic Concepts Related to Kinetics Clearly, the weight bar will stay in place in the absence of being lifted because of it’s inertia. Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-8 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) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-9 Force • Force: a push or pull in a direction. • F=mxa • Kinetic energy:1/2 mass x velocity² Basic Concepts Related to Kinetics What is a free body diagram? Force applied Ball being struck by a racquet Air resistance by racquet ball weight (diagram showing vector representations of all forces acting on a defined system) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-11 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 Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-12 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 Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-13 Basic Concepts Related to Kinetics What is a torque? T = Fd F = 10N T = (10N)(2m) T = 20 Nm d = 2m axis T = Fd (the product of force and the perpendicular distance from the force’s line of action to the axis of rotation) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-14 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 Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-15 Center of Gravity • Point at which all body’s mass and weight is evenly distributed in all directions. • Center of gravity is dependent upon: – Body shape (anthropometrics, gender) – Body position • Base of support Principles of balance and stability A person has balance … • When the c/g falls within the base • In direct proportion to size of the base • Depending on weight (mass). • Depending on height of c/g • Depending on how close c/g is to edge of base. • When c/g is nearer the center of the base. Principles of balance and stability • To meet an oncoming force, stability is increased by: – moving c/g toward the force. – Enlarging base on the side of the force • Equilibrium enhanced by increasing friction between body and surface it contacts. More principles of balance and stability • Rotation about an axis increases stability. • Locomotion (walking) is created by dislocating the c/g from the base and then reestablishing it within the base. • Movement in a direction may be enhanced by moving the c/g within the base in that direction. Basic Concepts Related to Kinetics 10N 20N CG 1m 2m The weights are balanced, creating equal torques on either side of the fulcrum. Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-20 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/s2) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-21 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 Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-22 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/m2 Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-23 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/m2 Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-24 Pressure and Stress • Pressure = force ÷ area • The greater the area over which a force is distributed, the lower the pressure (amount of stress). • Muscles and bones adapt to stress by hypertrophy. Basic Concepts Related to Kinetics What is stress? Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-26 Bone adaptations to stress • Hypertrophy: bones increase in density when subjected to continuous stress. • Bone atrophy and osteoporosis • Bony insertion points for tendon attachments (muscles) also increase in size to accommodate for increased tensile stress. Tendon and ligament responses to stress • Tendons and ligaments increase in size and strength to accommodate for increased tensile stress. Basic Concepts Related to Kinetics What is volume? • space occupied by a body • has three dimensions (width, height, and depth) • units are m3 and cm3 Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 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/m3 Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 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/m3 Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-31 Basic Concepts Related to Kinetics What is impulse? • the product of force and the time over which the force acts (Ft) • units are Ns Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-32 Mechanical loading • Types of forces: – Tension • Muscles pulling on bone. – Compression • Gravity compressing spine or long bones. – Shear • Getting hit in the side of the knee. – Bending • Greenstick fractures – Torsion • Twisting knee Basic Concepts Related to Kinetics What is compression? Original Shape Compression (pressing or squeezing force directed axially through a body) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-34 Basic Concepts Related to Kinetics What is tension? Original Shape Tension (pulling or stretching force directed axially through a body) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-35 Basic Concepts Related to Kinetics What is shear? Original Shape Shear (force directed parallel to a surface) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 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) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-37 Basic Concepts Related to Kinetics What is torsion? Neutral axis (load producing twisting of a body around its longitudinal axis) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-38 Basic Concepts Related to Kinetics What is deformation? Load Yield Point Elastic Region Ultimate Failure Point Plastic Region Deformation (change in shape) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-39 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 Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-40 Basic Concepts Related to Kinetics Load Magnitude Repetitive vs. acute loading Likelihood of Injury Frequency of Loading Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-41 Vector Algebra What is vector composition? (process of determining a single vector from two or more vectors by vector addition) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-42 Vector Algebra Vectors may be resolved into perpendicular components. The vector composition of each pair of components yields the original vector. Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 3-43