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Unformatted text preview: Niu (qn269) HW03 Tsoi (58020) 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 (part 1 of 3) 10.0 points Consider the following statement made re- garding 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 di- rected down. These are equal and opposite to one another. By New- tons third law they are an action- reaction pair, so the normal force is always equal to the weight of the book. Do you agree with the statement? 1. Agree; a normal force only exerts enough force to keep the object from falling through. The normal force and weight are always equal, even when other forces are present. 2. Disagree; the weight must be slightly greater than the normal force to keep the book in contact with the table. However, the forces are an action-reaction force pair because they are acting on one object. 3. Disagree; if an additional force acts down on the book, the normal force must also counter this extra force. correct 4. Agree; for every action there is an equal and opposite reaction (Newtons third law). Since gravity pulls down on the book, to be equal and opposite the force of gravity must equal the normal force. Explanation: If we were to put a weight on the book, the normal force would not be simply the weight of the book as the statement implies (see part 2). Note: This force pair is not a third law action-reaction pair. The force pairs in New- tons third law always act on two different objects! The action-reaction pair is Earth exerts a force on the book and the book exerts a force on the Earth. Caution: The explanation introduces some- thing not mentioned in the problem. 002 (part 2 of 3) 10.0 points Consider a book on top of a level table while the book is being pressed straight down with a force F by your hand. Book F The following figures show several attempts at drawing free-body diagrams for the book. Which figure has the correct directions for each force? Note: The magnitude of the forces are not necessarily drawn to scale. 1. hand normal gravitational 2. gravitational hand normal 3. normal gravitational hand correct Niu (qn269) HW03 Tsoi (58020) 2 4. hand normal gravitational 5. normal gravitational hand 6. gravitational hand normal Explanation: The normal force opposes both the gravita- tional force down and the F hand down. 003 (part 3 of 3) 10.0 points What forces change when comparing the free body diagram before the hand was placed on the book to after? 1. normal and F hand correct 2. F hand 3. gravitational, normal, and F hand 4. gravitational and F hand 5. normal 6. None of these 7. gravitational 8. gravitational and normal Explanation: There will be a change in both the nor- mal force and the F hand . When the hand is removed, the normal force is equal to the weight of the book....
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This note was uploaded on 09/01/2009 for the course PHY M taught by Professor Staff during the Spring '09 term at University of Texas at Austin.
- Spring '09