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Kinesiology 2241A/B Lecture 9: Lecture #9- Forces and Movement

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copperlizard489
1 Feb 2018
School
Western University
Department
Kinesiology
Course
Kinesiology 2241A/B
Professor
Thomas Richard Jenkyn

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Document Summary

If a force is applied to an object, that object pushes back with the same force: this pair of forces are the action force and the reaction force, they are equal in magnitude and opposite in direction. In biomechanics we"re generally talking about reaction forces acting on our body. Football example, the reaction force is what causes the acl to tear: reaction forces are what cause us to be able to jump upward. Internal vs external forces: the reaction forces that we have been talking about are external forces. Forces, internal or external, that act to accelerate a segment are motive forces: these are the forces that caused the motions in the body segments of the football kicker from lab. Force, as we define in last class, is an action that causes a mass to accelerate: a pressure, is a force divided by the (perpendicular) area over which the force acts, pressure =f/area, high pressure.

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Related Questions

A) A siring is tied to a book and pulled lightly as shown at right. The book remains in contact with the table and does not move. Draw a free-body diagram for the book. Label each of the forces exerted on the book as you were instructed in the tutorial Forces.

B) How do the forces exerted on the book, in this case, compare to the forces exerted on the book when the string is not present? List any forces that are the same (i.e., same type of force, same direction, and same magnitude) in both cases. Make a separate list of forces that change (or are not present) when the string is pulled.

C) Consider the following statement made by a student about a book at rest on a level table: "The two forces exerted on the book are the normal force directed up and the weight of the book directed down. These are equal and opposite to one another. By Newtons third law they are (action-reaction) force pair, so the normal force is always equal to the weight of the book." Do you agree with the student? Explain why you agree or disagree.

D) Consider a book on top of a level table while the book is being pressed straight down by a hand. In the space below, draw a free-body diagram for the book. Label the forces as you did in the tutorial Forces. How do the forces exerted on the book in this case compare to the forces exerted on the book when the hand is not pushing? List any forces that are the same (i.e., same type of force, same direction, and same magnitude) in both cases. Make a separate list of forces that change (or are not present) when the hand is pressing down on the book.

E) When the hand is pushing down on the book, is the magnitude of the weight equal to the magnitude of the normal force exerted by the table on the book? How can you tell?