Linear Kinetics of Human Movement Chapter 12 Linear Kinetics of Human Movement Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. © 2012 The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill/Irwin

Laws of motion Newton’s first law (law of inertia) SirIsaac Newton (1643-1727) 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?

Basic Biomechanics, 6th edition Newton’s Laws What is the law of inertia? A body will maintain a state of rest or constant velocity unless acted on by an external force that changes the state. Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Basic Biomechanics, 6th edition Newton’s Laws A skater has a tendency to continue gliding with constant speed and direction because of inertia. Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

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

Basic Biomechanics, 6th edition Newton’s Laws What is the law of acceleration? A force applied to a body causes acceleration of that body of a magnitude proportional to the force in the direction of the force and inversely proportional to the body’s mass F = ma Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Basic Biomechanics, 6th edition Newton’s Laws What is the law of reaction? For every action, there is an equal and opposite reaction. When one body exerts a force on a second, the second body exerts a reaction force that is equal in magnitude and opposite in direction on the first body. Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

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 Biomechanics, 6th edition Newton’s Laws In accordance with the law of reaction, the weight of a box sitting on a table generates a reaction force by the table that is equal in magnitude and opposite in direction to the weight. wt R Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Basic Biomechanics, 6th edition Newton’s Laws In accordance with Newton’s third law of motion, ground reaction forces are sustained with every footfall during running. Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Mechanical Behavior of Bodies in Contact What is friction? A force acting over the area of contact between two surfaces direction is opposite of motion or motion tendency magnitude is the product of the coefficient of friction () and the normal reaction force (R) F = R Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Mechanical Behavior of Bodies in Contact What is momentum? quantity of motion possessed by a body measured as the product of a body’s mass and its velocity; M = mv Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Mechanical Behavior of Bodies in Contact What is the principle of conservation of momentum? In the absence of external forces, the total momentum of a given system remains constant. M1 = M2 (mv)1 = (mv)2 Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Mechanical Behavior of Bodies in Contact What causes momentum? impulse: the product of a force and the time interval over which the force acts Ft Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Mechanical Behavior of Bodies in Contact What is impact? a collision characterized by: the exchange of a large force during a small time interval Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Mechanical Behavior of Bodies in Contact What happens following an impact? This depends on: the momentum present in the system the nature of the impact Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Mechanical Behavior of Bodies in Contact What happens during impact? This is described by the coefficient of restitution, a number that serves as an index of elasticity for colliding bodies; represented as e. Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Work, Power, and Energy Relationships What is mechanical work? the product of a force applied against a resistance and the displacement of the resistance in the direction of the force W = Fd units of work are Joules (J) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Work, Power, and Energy Relationships What is mechanical power? the rate of work production calculated as work divided by the time over which the work was done W P = t units of work are Watts (W) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Work, Power, and Energy Relationships What is mechanical energy? the capacity to do work units of energy are Joules (J) there are three forms energy: kinetic energy potential energy thermal energy Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Work, Power, and Energy Relationships What is kinetic energy? energy of motion KE = ½mv2 What is potential energy? energy by virtue of a body’s position or configuration PE = (wt)(ht) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Work, Power, and Energy Relationships During the pole vault, the bent pole stores potential energy for subsequent release as kinetic energy and thermal energy. Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

Work, Power, and Energy Relationships What is the law of conservation of mechanical energy? When gravity is the only acting external force, a body’s mechanical energy remains constant. KE + PE = C (where C is a constant - a number that remains unchanged) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.