PHYSICS 8A-02 (02-4-10)

# PHYSICS 8A-02 (02-4-10) - PHYSICS 8A Professor Joel Fajans...

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PHYSICS 8A Professor Joel Fajans 2/4/10 Lecture 6 ASUC Lecture Notes Online is the only authorized note-taking service at UC Berkeley. Do not share, copy or illegally distribute (electronically or otherwise) these notes. Our student-run program depends on your individual subscription for its continued existence. These notes are copyrighted by the University of California and are for your personal use only. D O N O T C O P Y Sharing or copying these notes is illegal and could end note taking for this course. ANNOUNCEMENTS I am going to poll the other class, and if they agree, I will agree the time that homework is due. The day will not change. Assume that it has not changed unless I tell you otherwise. LECTURE Today we begin by discussing Newton’s third law. We will continue to free body diagrams and friction, if we get far enough. The basic idea Newton’s third law is as follows: I am holding a weight and would like to know something about it. It is heavy and pushes on my hand. I need to push back up on the weight to keep it stationary. In other words, We draw a diagram and a free body diagram If I leave the mass on the table, what does the picture look like? There really is no difference, other than that there is no active actor. The table must be pushing up on the mass with the force equal to mg , otherwise the mass would fall through the table. In other words, for every force there is an equal and opposite force. This statement is known as Newton’s third law. Newton’s third law: For every force there is an equal and opposite force. This sounds simple enough, but it is useful to know. We are talking about forces involved in the same situation, not another force somewhere else. So when there is an equal and opposite force, when I am pushing up on the weight, it is pushing down on me, and these are equal and opposite. Let’s consider the act of me walking up the stairs. I just walked upwards. Where did the force come from, that made me go upwards? Imagine if I were

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PHYSICS 8A ASUC Lecture Notes Online: Approved by the UC Board of Regents 2/4/10 D O N O T C O P Y Sharing or copying these notes is illegal and could end note taking for this course. 2
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## This note was uploaded on 04/29/2010 for the course PHYSICS 83840p3 taught by Professor Fajans during the Spring '10 term at Berkeley.

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PHYSICS 8A-02 (02-4-10) - PHYSICS 8A Professor Joel Fajans...

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