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Why is acceleration positive in this exercise?

  1. Mar 6, 2016 #1
    1. The problem statement, all variables and given/known data
    Calculate the acceleration of a 1400kg car if it can stop from 35km/h (9.72m/s) on a dime (diameter= 1.7cm) (0.017m). What is the force felt by the 68kg occupant of the car?

    2. Relevant equations
    v^2=v0^2+2a(x-x0)
    F=ma

    3. The attempt at a solution
    ok after solving for a in that equation i get -2779m/s^2. when i go to calculate the force felt by the 68kg constituant, i'd just plug everything in, which would mean i'd get a negative force. the back of my book says that the answer is correct but instead of it being negative like i have it it is positive. WHY? why was the acceleration changed?
     
  2. jcsd
  3. Mar 6, 2016 #2
    It's difficult to tell without having the book, but it's likely that it was simply looking for the magnitude of the force, which would be positive.
     
  4. Mar 6, 2016 #3
    Capture.PNG the books question
    Capture2.PNG

    the answer
     
  5. Mar 6, 2016 #4
    Yes, it looks like they're just looking for the magnitude of the force.
     
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