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

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SUMMARY

The maximum acceleration a car can achieve, given a coefficient of static friction of 0.80, is determined using Newton's second law. The discussion highlights the importance of correctly identifying the direction of forces in a free body diagram. Specifically, the static friction force should be placed in the direction opposite to the acceleration of the car, which is positive when the car accelerates from rest. Misplacing the static friction force can lead to incorrect conclusions about acceleration signs.

PREREQUISITES
  • Understanding of Newton's second law of motion
  • Familiarity with free body diagrams
  • Knowledge of static friction and its coefficient
  • Basic concepts of acceleration and force direction
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  • Study the application of Newton's second law in various scenarios
  • Learn how to construct and analyze free body diagrams
  • Research the effects of different coefficients of friction on acceleration
  • Explore the relationship between force direction and acceleration in physics
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Students studying physics, particularly those focusing on mechanics, as well as educators looking for effective ways to teach concepts of force and acceleration.

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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?
 
Physics news on Phys.org
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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