408 Lab 6

408 Lab 6 - EXSC 408L Introduction to Biomechanics Lab 6 -...

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EXSC 408L Fall ‘11 Introduction to Biomechanics Lab 6 - Total Body Kinetics Page 1 of 7 Lab #6 - Total Body Kinetics Purpose: The purpose of this lab is to fully comprehend, through personal experience, the relationship between the net force acting on the body and the acceleration of the total body center of mass (TBCM). In this lab, we will focus on vertical ground reaction forces and the subsequent linear motion of the TBCM. Upon completion of this lab you will: Understand the relationship between force, mass, and acceleration. Know how to interpret a force-time curve. Be able to draw a free-body diagram and use it to calculate net force. Understand and be able to solve projectile motion problems. Introduction: In many skilled tasks, the performer needs to perform a series of movements while in contact with the ground and during flight. As we have learned in the previous labs, the mechanical objectives of the phases during foot contact are generally to control and generate the linear and angular momentum necessary for the subsequent flight phase. The final condition of the foot contact phase (velocity at take-off) becomes the input or initial condition of the flight phase. During the flight phase, the path of the total body center of mass (TBCM) follows the laws of projectile motion. I. NET FORCE = MASS * ACCELERATION In addition to Newton's third law of motion referred to as the Law of Action/Reaction, you need to understand Newton's second law of motion referred to as the Law of Acceleration. Newton's Second Law: Σ F = m*a where: Σ F = sum of the external forces m = mass a = acceleration This mathematical formula expresses the relationship between the external forces applied to an object and the linear acceleration that the forces produce. It must be noted that the “a” in the above equation is the acceleration of the TBCM. In addition, the acceleration must be in the direction of the net external force, F. Newton's second law of motion, in conjunction with his third law (which states that for every action there is an equal and opposite reaction), provides the mathematical basis for what happens when someone performs a movement on a force platform. Σ F = ma substitute in Σ F y = R y + (-BW) R y + (-BW) = ma R y = BW + ma where: BW = body weight of person R y = vertical ground reaction force For the case in which a person is in a stationary (static) position, the two forces (BW t and R y ) must be equal in magnitude and opposite in direction ( Σ F y = R y -BW = m*a = 0). If the person starts to move up (positive acceleration of the TBCM), then Σ F = ma > 0 and the magnitude of the ground reaction force will be greater than the magnitude of the body weight. With initiation of downwards movement (negative acceleration of the TBCM), Σ F = ma < 0 and the magnitude of the ground reaction force will be less than the magnitude of the body weight.
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EXSC 408L Fall ‘11 Introduction to Biomechanics Lab 6 - Total Body Kinetics
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This note was uploaded on 12/04/2011 for the course EXSC 408L at USC.

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408 Lab 6 - EXSC 408L Introduction to Biomechanics Lab 6 -...

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