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Homework Help: Newton's Laws: Finding the tension in a cord.

  1. Sep 23, 2007 #1
    1. The problem statement, all variables and given/known data
    A 5.0-kg mass hangs at the end of a cord. Find the tensionin the cord if the acceleration of the mass is a)1.5 m/s squared up, b) 1.5 m/s squared down

    Answers: a) 57 N; b)42 N

    A sample question in our physics book is "An object of mass m is supported by a cord. Find the tension in the cord if the object is a) at rest, b) moving at a constant velocity, c) accelerating upward with acceleration a = 3g/2, and d) accelerating downward at a = 0.75g

    a) ay = 0: FT - mg = may = 0 or FT = mg
    b) ay = 0: FT - mg = may = 0 or FT = mg
    c) ay = 3g/2: FT - mg = m(3g/2) or FT = 2.5mg
    d) ay = -3g/4: FT - mg = m(-3g/2)or FT = 0.25mg

    2. Relevant equations
    The relevant equations are:

    See 1

    3. The attempt at a solution
    I know there is something I'm just not getting in this problem... It should be so simple but everytime i look at it and attempt it i just keep getting the wrong answer... Also in the sample problem, how are they getting 2.5 out of 3g/2??? It's just not clicking!!!
  2. jcsd
  3. Sep 23, 2007 #2


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    Remember f = ma and forces add so you have weight f = m g, and an extra force due to accelration f = m a.
    Think about wether acclerating up or down will make the tension more or less to tell you if you should add or subtract the second force.
  4. Sep 23, 2007 #3
    so basically all you do is add?

    (mg) + (ma) = (5 x 9.8) + (5 x 1.5) = 56.5 ~57 N
    (mg) + (ma) = (5 x 9.8) + (5 x -1.5) = 41.5 ~42 N
    (mg) + (ma) = (5 x 9.8) + (5 x -9.8) = 0

    ahh it makes so much sense now! i was so fixed on only using one equation. i never thought of using the 2 together. i was thinking too simple now. welli suppose i can blame my teacher for telling us to think simple. thank you!
  5. Sep 23, 2007 #4


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    The trick to classical physics is
    1, draw a diagram
    2, don't do the maths until you understand whats happening
    3, it's generally simpler than you think
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