Homework 4, Newton's laws 23-24-solutions

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levesque (aml6939) – Homework 4, Newton’s laws 23-24 – tejeda – (WilkesDPHY1 1) 1 This print-out should have 24 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. You will have approximately two weeks to complete this homework assignment. Collab- oration on homework is encouraged. How- ever, you must submit your own responses, and you may be required to show your in- dependent work to your HS Instructor. You are expected to access Quest daily to progress through this homework assignment. 001(part1of2)10.0points A book is at rest on an incline as shown below. A hand, in contact with the top of the book, produces a constant force F hand vertically downward. F hand Book The following figures show several attempts at drawing free-body diagrams for the book. Which figure has the correct directions for each force? The magnitudes of the forces are not necessarily drawn to scale. 1. F g F hand F N F f 2. F g F hand F N F f 3. F g F f F N F hand 4. F N F f F g F hand 5. F g F N F f F hand 6. F N F hand F f F g 7. F g F f F hand F N
levesque (aml6939) – Homework 4, Newton’s laws 23-24 – tejeda – (WilkesDPHY1 1) 2 8. F g F hand F f F N correct Explanation: The normal force points perpendicular to the surface of the inclined plane. The weight force points down. The F hand also points down. The friction force keeps the book from sliding and consequently points up the incline. 002(part2of2)10.0points For the normal force exerted on the book by the wedge in the diagram, which force(s) complete(s) the force pair for Newton’s third law (action-reaction)? 1. the component of gravity pointing parallel to the surface of the incline 2. the pull of the earth on the book 3. the component of F hand pointing perpen- dicular to the surface of the incline 4. the normal force exerted on the wedge by the book correct 5. the pull of the book on the earth 6. the sum of the component of gravity per- pendicular to the surface of the incline and the component of F hand perpendicular to the surface of the incline 7. the component of gravity pointing per- pendicular to the surface of the incline Explanation: The force that completes the third law pair with the normal force of the wedge on the book is the normal force of the book on the wedge. The “the sum of the component of gravity perpendicular to the surface of the incline and the component of F hand perpendicular to the surface of the incline” is equal and opposite to the normal force, but it is not a action-reactionpair ; e.g. , two forces equal and opposite to each other. 003(part1of2)10.0points A balloon is waiting to take off. As seen in the figure below, the balloon’s basket sits on a platform which rests on the ground. The balloon is pulling up on the basket, but not hard enough to lift it off the platform. platform basket ground balloon What is the free-body diagram for the plat- form? Some parallel vectors are offset hori- zontally for clarity. 1. F ground on plat F grav on plat F bask on plat correct 2. F bal on bask F grav on bask F grav on plat 3. F bal on plat F grav on plat 4. F bal on plat F plat on ground F grav on plat 5. F plat on bask F plat on ground F grav on plat Explanation:
levesque (aml6939) – Homework 4, Newton’s laws 23-24 – tejeda – (WilkesDPHY1 1) 3 All forces acting on the platform must be represented. No forces acting on other objects should be represented. 004(part2of2)10.0points What is the free body diagram for the bas- ket? Again some parallel vectors are offset horizontally for clarity. 1. F bal on bask F grav on bask 2. F grav on bask F bal on bask F plat on bask correct 3. F bask on plat F bask on bal F grav on bask 4. F bask on plat F bask on bal F bal on bask 5. F bal on bask F bal on bask F ground on bask Explanation: All forces acting on the basket must be represented. No forces acting on other objects should be represented. 005 10.0points You place a box weighing 214 . 7 N on an in- clined plane that makes a 35 . 2 angle with the horizontal. Compute the component of the gravita- tional force acting down the inclined plane. Answer in units of N. Correct answer: 123 . 76 N. Explanation: Basicconcepts θ W θ θ W W 1 The component of the gravitational force acting down the plane is the side opposite the angle θ , so W 1 = W sin θ 006 10.0points If an object is not accelerating, how many forces act on it? 1. 2 2. 1 3. 0 4. Unable to determine correct 5. 3 Explanation: If it isn’t accelerating, no net force acts on it; you do not know how many forces are involved. 007(part1of2)10.0points A gust of wind blows an apple from a tree. As the apple falls, the force of gravity on the apple is 9.27 N downward, and the force of the wind on the apple is 1.13 N to the right. What is the magnitude of the net external force on the apple? Answer in units of N. Correct answer: 9 . 33862 N. Explanation: Let : F y = - 9 . 27 N and F x = 1 . 13 N .
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