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

http://www.mhhe.com/hall4e

– 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 436-442)• Kinematics of projectile motion (pp 329-347)

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

Quantity Symbol Metric Unit English UnitMass m kg slugForce F N lbPressure P Pa psiVolume (solids) V m3 ft3

(liquids) liter gallonDensity kg/m3 lb/ft3

Specific weight N/m3 lb/ft3

Torque T N-m ft-lbImpulse 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 As = 4 cm2 Ac = 175 cm2

SolutionWanted: AnswerPressure 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

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 cm2

Formula: Stress = F/A

Stress = (625 N) (0.45) / 20 cm2

Stress = 14 N / cm2

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 cm2

Formula: Stress = F/A

Stress = (625 N) (0.45) + 222 N / 20 cm2

Stress = 25.2 N / cm2

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• DeformationWhen an external force is applied to the human body,

several factors influence whether an injury occursMagnitude 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 software

• Dynamography– Primarily employed in gait research– Starts, takeoffs, landings, baseball & golf swings, and

balance

Force Platform – Measurement of ground reaction forces