Transcript Chapter 3
Chapter 3: Kinetic Concepts for Analyzing Human Motion Basic Biomechanics, 4th edition Susan J. Hall Presentation Created by TK Koesterer, Ph.D., ATC Humboldt State University Objectives • Define and identify common units of measurement for mass, force, weight, pressure, volume, density, specific weight, torque, and impulse • Identify and describe the different types of mechanical loads that act on the human body • Identify and describe the uses of available instrumentation for measuring kinetic quantities • Distinguish between vector and scalar quantities • Solve quantitative problem involving vector quantities using both graphic and trigonometric procedures Basic Concepts Related to Kinetics • Inertia • Mass • Force – Free body diagram • Center of Gravity • Weight • • • • • • Pressure Volume Density Specific weight Torque Impulse Sample Problem 1 1. If a scale shows that an individual has a mass of 68 kg, what is that individual’s weight? Known: m = 68 kg Solution Answer Wanted: weight wt = 667 N Formulas: wt = mag wt = 150 lbs 1 kg = 2.2 lbs Sample Problem 1 2. What is the mass of an object weighting 1200 N? Known: wt = 1200 N Solution Answer Wanted: mass m = 122.32 kg Formulas: wt = mag Sample Problem 2 Is it better to be stepped on by a women wearing a spike or by a court shoe? Known: wt = 556 N As = 4 cm2 Ac = 175 cm2 Solution Wanted: Answer Pressure exerted by the spike heel p = 139N/cm2 Pressure exerted by the court shoe p = 3.8 N/Cm2 Formulas: p = F/A 43.75 times more pressure Common Units for Kinetic Quantities Quantity Symbol Mass m Force F Pressure P Volume (solids) V (liquids) Density Specific weight Torque T Impulse Metric Unit English Unit kg slug N lb Pa psi m3 ft3 liter gallon kg/m3 lb/ft3 N/m3 lb/ft3 N-m ft-lb N•s lb • s Mechanical Loads on the Human Body • • • • • • • Compression Tension Shear Stress Torsion Bending Combined loading Sample Problem 3 • How much compressive stress is present on the L1, L2 vertebral disk of a 625 N woman, given that approximately 45% of body weight is supported by the disk Sample Problem 3 a) When she stands in anatomical positions? Given: F = (625 N) (0.45) A = 20 cm2 Formula: Stress = F/A Stress = (625 N) (0.45) / 20 cm2 Stress = 14 N / cm2 Sample Problem 3 b) When she stands erect holding a 222 N suitcase? Given: F = (625 N) (0.45) = 222 N A = 20 cm2 Formula: Stress = F/A Stress = (625 N) (0.45) + 222 N / 20 cm2 Stress = 25.2 N / cm2 The Effects of Loading • Deformation When an external force is applied to the human body, several factors influence whether an injury occurs – Magnitude and direction of force – Area over which force is distributed – Load-deformation curve – Yield point (elastic limit) – Failure Repetitive vs. Acute Loads • • • • Repetitive loading Acute loading Macrotrauma Microtrauma Tools for Measuring Kinetic Quantities • Electromyography (EMG) – To study neuromuscular function • Dynamography – Primarily employed in gait research – Starts, takeoffs, landings, baseball & golf swings, and balance Vector Algebra • Vector • Kinetic vector quantities – force weight, pressure, specific weight & torque • Kinematic vector quantities – Displacement, velocity & acceleration • Scalar quantities – Mass, volume, length & speed Vector Composition • Resultant vector • “Tip-to-tail” vector composition Resultant vector Vector #2 Vector #1 Vector Resolution Example: A ball is thrown into the air Vertical Horizontal Graphic Solution of Vector Problems • Graphic vector manipulation may yield approximate result 1 cm = 10 N 30 N = 3 cm 35 N = 4.5 cm Trigonometric Solution of Vector Problems • A more accurate procedure for quantitatively dealing with vector problems Summary • This chapter introduced basic concepts related to kinetics • Several types of mechanical loads act on the human body. • The distribution of force within a body structure is termed mechanical stress. • Vectors quantities have magnitude & direction • Vector problems may be solved by a graphic or a trigonometric approach.