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

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To determine the maximum acceleration of a car with a static friction coefficient of 0.80, Newton's second law is applied. The main issue discussed is the placement of the static friction force in the free body diagram, which affects the sign of the acceleration. The user initially placed the static friction force on the negative side of the x-axis, resulting in a negative acceleration, contrary to the expected positive value. Clarification is sought on how to correctly identify the direction of forces, particularly the friction force acting on the tires during acceleration. Understanding the correct orientation of forces is crucial for accurate calculations in physics problems.
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Homework Statement


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?


Homework Equations



I used Newton's 2nd laws and solved

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?
 
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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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