Week two overview Wednesday (Jan 29) –Kinetic concepts (Read Ch 3 of Hall) Basic concepts related to kinetics Mechanical loads on the human body Tools for measuring kinetic quantities Take text web-based practice exam –Monday (February 3) Vector resolution and summation (pp 76-80) Center of mass – what it is and how to determine it in the human body (pp ) Kinematics of projectile motion (pp )

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 Solve quantitative problem involving segmental calculation of total body COM Identify and describe kinematic characteristics of projectile motion

Basic Concepts Related to Kinetics Inertia Mass Force –Free body diagram Center of Gravity Weight Pressure: Force/Area Volume: LWH Density: Mass/Vol Specific wt: Wt/Vol Torque: Fd Impulse: Ft

Common Units for Kinetic Quantities QuantitySymbolMetric UnitEnglish Unit Mass m kg slug Force F N lb Pressure P Pa psi Volume (solids) V m 3 ft 3 (liquids) liter gallon Density  kg/m 3 lb/ft 3 Specific weight  N/m 3 lb/ft 3 Torque T N-m ft-lb Impulse N s lb s

All About Forces What is a force? Why study forces? Four properties of a force Mass, gravity, and weight Center of gravity Questions: –If you are standing in the anatomical position and you raise your ams overhead, in what direction does you center of mass move? – If you raise your arms to the front, why do you lean backward?

Four Properties of forces: Center of Mass of different objects:

Free body diagrams:

Sample Problem 1 on pressure Is it better to be stepped on by a women wearing a spike or by a court shoe? Known: wt = 556 N A s = 4 cm 2 A c = 175 cm 2 Solution Wanted: Answer Pressure exerted by the spike heel p = 139N/cm 2 Pressure exerted by the court shoe p = 3.8 N/Cm 2 Formulas: p = F/A times more pressure

Sample Problem on Pressure 2 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 on pressure 2 a)When she stands in anatomical positions? Given: F = (625 N) (0.45) A = 20 cm 2 Formula: Stress = F/A Stress = (625 N) (0.45) / 20 cm 2 Stress = 14 N / cm 2

Sample Problem on pressure 2 b) When she stands erect holding a 222 N suitcase? Given: F = (625 N) (0.45) = 222 N A = 20 cm 2 Formula: Stress = F/A Stress = (625 N) (0.45) N / 20 cm 2 Stress = 25.2 N / cm 2

Mechanical Loads on the Human Body Compression Tension Shear Torsion

Bone loading modes: Compression – pushing together Tension – pulling apart Torsion – twisting Shear – cutting across

Cutting across

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

Load- deformation relationship: Stress-strain curve:

Repetitive vs. Acute Loads Repetitive loading Acute loading Macrotrauma Microtrauma

Tools for Measuring Kinetic Quantities Electromyography (EMG) –To study neuromuscular function –Website: Surface electromyography systems, emg electrode, emg acquisition analysis softwareSurface electromyography systems, emg electrode, emg acquisition analysis software Dynamography –Primarily employed in gait research –Starts, takeoffs, landings, baseball & golf swings, and balance

Force Platform – Measurement of ground reaction forces