Kinetic Concepts for Analyzing Human Motion

Kinetic Concepts for Analyzing Human Motion
Chapter 3 Kinetic Concepts for Analyzing Human Motion Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D. © 2014 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill/Irwin

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 Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

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 kg (1.37 lb.). (Mass is proportional to weight but note that mass is not related to size.) Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

What is inertia? Tendency to resist change in state of motion Proportional to mass Has no units! Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

Clearly, the weight bar will stay in place in the absence of being lifted because of its inertia. Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

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, 7th edition By Susan J. Hall, Ph.D.

While there is no motion occurring, boy and dog are exerting equal and opposite forces on the rope. Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

What is a free body diagram? Ball weight Force applied by racquet Air resistance Ball being struck by a racquet (Diagram showing vector representations of all forces acting on a defined system) Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

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, 7th edition By Susan J. Hall, Ph.D.

What is a torque (T)? The rotary effect of a force The angular equivalent of force Also known as moment of force Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

What is a torque? T = Fd T = (10N)(2m) T = 20 Nm F = 10N 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, 7th edition By Susan J. Hall, Ph.D.

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, 7th edition By Susan J. Hall, Ph.D.

1m 2m CG The weights are balanced, creating equal torques on either side of the fulcrum. Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

If one child on a see-saw is heavier than the other, how can they balance? Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

What is weight? Attractive force that the earth exerts on a body wt. = mag (product of mass and the acceleration of gravity: m/s2) Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

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, 7th edition By Susan J. Hall, Ph.D.

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, 7th edition By Susan J. Hall, Ph.D.

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, 7th edition By Susan J. Hall, Ph.D.

What is stress? Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

Why would it hurt more to be stepped on by someone in high heels as compared to the same person wearing athletic shoes? Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

What is volume? Space occupied by a body Has three dimensions (width, height, and depth) Units are m3 and cm3 Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

What is density? Mass per unit of volume Represented with the small Greek letter rho:  Units are kg/m3 Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

What is specific weight? Weight per unit of volume Represented with the Greek letter gamma:  Units are N/m3 Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

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. Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

What is impulse? The product of force and the time over which the force acts (Ft) Units are Ns Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

What is compression? Original shape Compression (Pressing or squeezing force directed axially through a body) Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

What is tension? Original shape Tension (Pulling or stretching force directed axially through a body) Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

What is shear? Original shape Shear (Force directed parallel to a surface) Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

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, 7th edition By Susan J. Hall, Ph.D.

What is torsion? Neutral axis (Load producing twisting of a body around its longitudinal axis) Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

What is deformation? Deformation Plastic Region Ultimate Failure Point Yield Elastic Load (Change in shape) Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

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, 7th edition By Susan J. Hall, Ph.D.

Repetitive vs. acute loading Likelihood of Injury Frequency of Loading Load Magnitude Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

Basic Biomechanics, 7th edition
Vector Algebra What is vector composition? (Process of determining a single vector from two or more vectors by vector addition) Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

Vector Algebra The composition of vectors with the same direction requires adding their magnitudes. Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

Vector Algebra The composition of vectors with the opposite directions requires subtracting their magnitudes. Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

Vector Algebra The tip-to-tail method of vector composition. Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

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) Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

Vector Algebra Vectors may be resolved into perpendicular components. The vector composition of each pair of components yields the original vector. Basic Biomechanics, 7th edition By Susan J. Hall, Ph.D.

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