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Calculating mass of a block

  1. May 4, 2010 #1
    Calculating mass of a block....

    1. The problem statement, all variables and given/known data

    Physicsquestions1.jpg

    2. Relevant equations

    Force equations/Kinetic equations


    3. The attempt at a solution

    FA = mA * 9.8 m/sec2

    FA = FB
    where FB is the force acting through the string from A to B

    The forces on B are friction from the table, friction from C, and the force it takes to move B's mass at 0.8 m/sec2
    FB = FfB + FfC + (1.4 kg * 0.8 m/sec2)

    FfC = 0.3 * ( 0.6 kg * 9.8 m/sec2 )

    FfC = 1.764 N

    FfB = 0.25 * [ (1.4 kg + 0.6 kg) * 9.8m/sec2 ]

    FfB = 4.9 N

    FB = 1.764 N + 4.9 N + (1.4 kg * 0.8 m/sec2)

    FB = 7.784 N

    FA = 7.784 N

    7.784 N = mA * 9.8 m/sec2

    mA = 0.794286 kg ~ 794 grams
     
  2. jcsd
  3. May 4, 2010 #2
    Re: Calculating mass of a block....

    No one?
     
  4. May 4, 2010 #3

    PhanthomJay

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    Re: Calculating mass of a block....

    This is not FA (the string tension), this is the weight of block mA. FA must be less than the weight of block mA, or else it can't accelerate downward.
    yes, this is correct.
    stay away from pseudo forces, the horizontal forces on B are friction from the table, friction from C, and the tension force FB
    yes...
    I'm not checking your numbers, but your equations are OK
    This is your error as i noted above; the weight mA acts down, and FA acts up, solve for mA using Newton 2.
     
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