classnote_signs - MECH 321 Notes: The sign convention in...

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©2005 Dong Qian All Rights Reserved 1 MECH 321 Notes: The sign convention in mechanical system Many times we found that the signs are confusing. After many years of struggling, I would like to provide you some simple ways of determining the signs, which will be shown below. I hope these shall serve the purpose of not having to spending a tremendous amount of time on it again. When we are trying to describe a system, you need to keep in mind that there are two things, one is the physics (or the phenomenon itself), the other is how we describe them. The physics itself is objective, while how we describe them depends on the standard that we establish for the purpose of getting the description. The trouble is that sometimes we can have multiple standards, then the question is how to unify or make the connection among these standards. Sign is contained in one of these standards. To make it easier to understand, I will use the unit as an analogy. For instance, 1 inch in the English system is 2.45 cm in the SI system. Although different number is used in different system, you know physically it refers to a length that is comparable to 1/12 of a foot, or 1/36 of a yard, and one yard is the average length of a stride. So we know what it is. In the signs we have learned so far, there are two systems (or two standards by analogy). I will call the first system a “ global equilibrium ” system. In this system, the direction of the
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classnote_signs - MECH 321 Notes: The sign convention in...

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