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Homework Help: Finding tension of a pulley

  1. Nov 2, 2007 #1
    1. The problem statement, all variables and given/known data

    A 957 g mass is attached to a pulley and a 8 N weight is attached to a thin massless cord.
    Pulley is massless and frictionless.
    Acceleration of gravity is 9.8 m/s^2

    http://img474.imageshack.us/img474/2863/16gs4.jpg [Broken]

    What is the tension T? Answer in N

    2. Relevant equations

    Fg (also weight) = ma

    1 gram = 0.001 kg

    3. The attempt at a solution

    957 g = 0.957 kg

    8 N = m * 9.81
    mass of block = 0.81549 kg

    w = 0.957 kg * 9.81
    w = 9.38817 N

    w = 0.81549 kg * 9.81
    w = 7.9999569 N

    Tension therefore is 9.38817 + 7.9995469 = 17.387739 N

    Did I interpret and did this right? Please check work
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Nov 2, 2007 #2


    User Avatar
    Science Advisor

    Do you notice that you got that "0.81549 kg" by dividing by 9.81 and then just multiplied by 9.81 again? Why not just use the 8 N you were given initially?

    Why are you giving the answer to 8 significant figures when you were only given the info to 3 significant figures? I would have calculated that weight of the 0.957 kg mass to be 9.39 N and then said 8.00+ 9.39= 17.39 or 17.4 N.
    (The fact that you measure one mass as 957 grams implies you are able to measure mass [and so weight] to three significant figures so I am interpreting "8 N" as 8.00 N.)

    Are you given or are you assuming that everything is static- that neither of the masses is moving?
    Last edited by a moderator: May 3, 2017
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