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Finding mass given force and acceleration

  1. Jan 25, 2012 #1
    a lift ascends with a uniform acceleration of 2m/s^2 whilst transporting a woman. Calculate her mass if the force exerted on her from the floor was 945N.

    ANS: 80KG

    I have tried F = MA

    M = F/A = 945/2 = 472.5

    Inertia may have been taken into account but is this possible without being given the mass?
  2. jcsd
  3. Jan 25, 2012 #2


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    Homework Helper

    I hate elevator questions; very tricky!
    You have the upward force of 945 N on the lady.
    But you forgot gravity also has a grip on her.
    Include that, and you will get the 80 kg!
  4. Jan 25, 2012 #3
    I hate them too! how do i include gravity in there? It would usually be the product of her mass and 9.81 but im not given her mass
  5. Jan 25, 2012 #4


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    Write it as mg. You'll have two m's in your F = ma equation but it is not a big problem to collect them as like terms in a linear equation. This is an important idea: write it out even if it strikes you as unsolvable - once it appears in front of you in a mathematical form, a whole different thought process in your mind takes over and often the difficulties disappear.
  6. Jan 26, 2012 #5
    945 - mg - ma = 0
    g + a x m = 945
    9.81 + 2 x m = 945
    m = 945/11.81 = 80kg
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