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Chapter 8 Skill Building

# Chapter 8 Skill Building - MasteringPhysics Assignment...

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MasteringPhysics: Assignment Print View Newton's 3rd Law Discussed Learning Goal: To understand Newton's 3rd law, which states that a physical interaction always generates a pair of forces on the two interacting bodies. In Principia , Newton wrote: To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. (translation by Cajori) The phrase after the colon (often omitted from textbooks) makes it clear that this is a statement about the nature of force. The central idea is that physical interactions (e.g., due to gravity, bodies touching, or electric forces) cause forces to arise between pairs of bodies. Each pairwise interaction produces a pair of opposite forces, one acting on each body. In summary, each physical interaction between two bodies generates a pair of forces. Whatever the physical cause of the interaction, the force on body A from body B is equal in magnitude and opposite in direction to the force on body B from body A. Incidentally, Newton states that the word "action" denotes both (a) the force due to an interaction and (b) the changes in momentum that it imparts to the two interacting bodies. If you haven't learned about momentum, don't worry; for now this is just a statement about the origin of forces. Mark each of the following statements as true or false. If a statement refers to "two bodies" interacting via some force, you are not to assume that these two bodies have the same mass. Part A Every force has one and only one 3rd law pair force. ANSWER: true false Part B The two forces in each pair act in opposite directions. ANSWER: true false Part C The two forces in each pair can act on the same body or on different bodies. ANSWER: true false Part D http://session.masteringphysics.com/myct/assignmentPrint?assignmentID=1043407 (1 of 22)4/11/2006 2:54:36 PM

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MasteringPhysics: Assignment Print View The two forces in each pair may have different physical origins (for instance, one of the forces could be due to gravity, and its pair force could be a normal contact force). ANSWER: true false Part E The two forces of a 3rd law pair always act on different bodies. ANSWER: true false Part F Given that two bodies interact via some force, the accelerations of these two bodies have the same magnitude but opposite directions. (Assume no other forces act on either body.) Hint F.1 Hint not displayed ANSWER: true false Newton's 3rd law can be summarixed as follows: A physical interaction (e.g., gravity) operates between two interacting bodies and generates a pair of opposite forces, one on each body. It offers you a way to test for real forces (i.e., those that belong on the force side of )--there should be a 3rd law pair force operating on some other body for each real force that acts on the body whose acceleration is under consideration.
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