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Homework Help: F=ma and Velocity equation

  1. Dec 13, 2011 #1
    1. The problem statement, all variables and given/known data
    A 12 kg box slides across the floor with an initial velocity of +8.4 m/s. It comes to a stop after sliding +13.8 m.

    How much friction was present?

    2. Relevant equations
    f=ma v^2=v(o)^2+2ad

    3. The attempt at a solution
    I did the ''v^2" equation to find out what the acceleration was, so i could find out what F equaled in f=ma. I got -2.55=a. I plugged that into F=ma, F=877(-2.55), which then F=-2242.06. But can a force be negative? And also from there I'm completely lost.
  2. jcsd
  3. Dec 13, 2011 #2
    Frictional force will be negative if you choose the displacement as positive because it is acting opposite to the direction of motion.

    Check your math. Where did you get 877 for the mass?? It says 12kg.
  4. Dec 13, 2011 #3
    Well. There we go it worked. I have not a clue where I got that 877. Sometimes another pair of eyes helps a lot haha
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