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Simple question regarding vectors in examples of physics textbooks

  1. Nov 8, 2014 #1
    I have noticed that in my classical mechanics course and in the textbook I read for it, the portion in examples that require to look at the sum of forces for the system is different than what I am accustomed to in introductory texts. For example, if we were dealing with a system with a ball of mass ##m## tied to a ceiling and were asked to find the forces, in an introductory textbook (as least the ones I read) would state the summation of forces as ##F_{net}=T-mg##. However, in the mechanics textbook it is written as ##F_{net}=T+mg##. I'm wondering why it seems that the downward aspect of the gravitational force is ignored. I figure that overall it doesn't make a difference, but I'm just curious for the author's reasoning as it leads me confused in both reading and comparing answers for exercises from the textbook.
    Last edited: Nov 8, 2014
  2. jcsd
  3. Nov 8, 2014 #2


    Staff: Mentor

    Try reading the textbook more carefully to understand how T and Fnet are defined the notion of what direction is positive is critical to understanding the equation.
  4. Nov 8, 2014 #3
    Chances are that the text that type F=T+Mg, is using a notation where F, T and Mg are vectors. fnet.jpg
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