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Homework Help: Maximum Tension

  1. Sep 18, 2009 #1
    1. The problem statement, all variables and given/known data

    A 1.49 kg mass is suspended from a string which is pulled upward. The mass accelerates upwards with an acceleration of 3.10 m/s2. What is the tension in the string?

    2. Relevant equations

    Fnet= T + Fg =ma


    T = m(a + g)

    3. The attempt at a solution

    Ok so I pluged in the numbers

    T = (1.49kg)(3.1m/s2+(9.89m/s2)

    and I got 19.221 kN but its wrong.

    The title for this is wrong I was going to ask another question originally.
  2. jcsd
  3. Sep 18, 2009 #2
    3.1 m/s^2 is accelerating the mass upwards. Ask yourself, what direction is the 9.81 m/s^2 acting in? Draw a free body diagram of the forces acting on the mass.

  4. Sep 18, 2009 #3
    I know gravity is downwards but there is a problem in the book just like this and this is how they get to that equation.

    T + Fg = ma

    so since Fg is negative

    T - mg =ma

    T = ma+mg

    T = m(a+g)

    I know that gravity was negative but thats why they added mg over to the other side.
  5. Sep 18, 2009 #4
    Yes, sorry I looked right past that. Not sure where you went wrong then. Are you close to the correct answer?

  6. Sep 18, 2009 #5
    It just says 19.221 kN is wrong it doesen't tell me anything else. Are my units right? I don't get why im getting this wrong.
  7. Sep 18, 2009 #6
    The units are correct. N = kg*m/s^2.

    Not sure why it isn't right. Maybe someone can help us both out. LOL

  8. Sep 18, 2009 #7
    I was getting it wrong because I was putting kN instead of N. I missed that the book converted their answer to kN, Thanks I needed to see N = kg*m/s^2 to catch my mistake.
  9. Sep 18, 2009 #8
    LOL geeze I missed it too. Whoops.

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