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Calculating Force of tension

  1. Dec 16, 2009 #1
    1. The problem statement, all variables and given/known data
    The following is for a lab report. We had a couple pulleys set up, with a weight on one side, and a weight on the other side. We timed how long it took for it to fall and figured out the acceleration was .891 m/s[tex]^{2}[/tex]
    http://img682.imageshack.us/img682/5316/uploadyt.jpg [Broken]
    I need to calculate the Force of tension for 50.0g mass, and the Force of tension for 60.0g mass. Gravity is 9.80 m/s[tex]^{2}[/tex]

    2. Relevant equations
    F[tex]_{tension} + F_{weight}[/tex] = mass * acceleration

    3. The attempt at a solution
    F[tex]_{tension} + F_{weight}[/tex] = mass * acceleration
    F[tex]_{tension}[/tex] = mass * acceleration - F[tex]_{weight}[/tex]

    F[tex]_{weight}[/tex] = mass * gravity
    F[tex]_{tension}[/tex] = (mass * acceleration) - (mass * gravity)
    F[tex]_{tension}[/tex] = (mass * .891 m/s[tex]_{2}[/tex]) + (mass * -9.80 m/s[tex]_{2}[/tex])

    Solving for 50.0g...
    50.0g = .0500kg
    F[tex]_{tension}[/tex] = (.0500kg * .891 m/s[tex]_{2}[/tex]) - (0.0500kg * -9.80 m/s[tex]_{2}[/tex])
    F[tex]_{tension}[/tex] = .535 N

    Solving for 60.0g...
    60.0g = .0600kg
    F[tex]_{tension}[/tex] = (.0600kg * .891 m/s[tex]_{2}[/tex]) - (0.0600kg * -9.80 m/s[tex]_{2}[/tex])
    F[tex]_{tension}[/tex] = .641 N

    However, apparently, one of the tension forces should be negative. I'm stumped at this point. Sorry if it seems to be a very basic mistake, but my teacher doesn't always explain things clearly, and all my classmates are equally confused at this point.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Dec 16, 2009 #2


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    Homework Helper

    You've defined the upward direction to be positive, and downward negative, right? I assume that's why you set g to a negative value. (Which is perfectly fine) In that case, both the tension forces should be positive because they're both pointing upward.

    Tension is normally given as a positive value, anyway, since we consider it to be the amount of force pulling pieces of the string together. A negative tension, to my mind, would mean that the string should spontaneously blow itself apart.
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