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Homework Help: Lifting a mass problem

  1. Jun 11, 2009 #1
    1. The problem statement, all variables and given/known data
    A 1.65 kg mass is suspended from a string which is pulled upward. The mass accelerates upwards with an acceleration of 3.10 m/s^2. What is the tension in the string?

    2. Relevant equations

    f = ma

    3. The attempt at a solution
    seems really easy but i can't get the right answer

    the regular tension would be 1.65kg * 9.8m/s^2 = 16.17N

    if its pulled up 3.1 m/s^2, the forces in that would be 1.65kg * 3.1m/s^2 = 11.05N

    then i subtracted the forces to get 5.1, but its wrong.

    that wasn't right so i added the forces, still wrong.

    then i did the acceleration as a whole, as in 9.8 + 3.1 = 12.9
    then found the forces which is 21.3N... still wrong

    anyideas? thanks
  2. jcsd
  3. Jun 11, 2009 #2
    Draw a free body diagram, and solve the problem as you see the forces there (gravity, upward acceleration included). The acceleration would be modified.
  4. Jun 11, 2009 #3


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    Homework Helper

    yes regular if it was in equilibrium, so just know that is the weight of the mass.

    3.1 m/s2 is the resultant acceleration, multiplying it by the 1.65kg gives the resultant force.

    In your free body diagram, the tension acts upwards while the weight acts downwards. Can you make a relationship between the tension,weight and resultant force? If you can do that, then the question become quite easy.
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