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How to determine what sign (+/-) to use?

  1. Feb 14, 2013 #1
    1. The problem statement, all variables and given/known data
    General question, but specific question here: "What is the maximum acceleration a car can undergo if the coefficient of static friction between the tires and ground is 0.80?


    2. Relevant equations

    I used Newton's 2nd laws and solved

    3. The attempt at a solution

    I guess my real issue is knowing which side of the free body diagram to place a force. I placed the static friction force on the negative side of the x-axis, and ended up with -F_s (static friction) = ma , which obviously leads to a negative acceleration. The answer is a positive acceleration, but I'm just not sure how to tell what side it should be on. Does it even make a difference?
     
  2. jcsd
  3. Feb 14, 2013 #2

    haruspex

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    A car accelerates from rest. Which way does the force of friction act on the tires? (Hint: what other horizontal forces are present?)
     
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