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Dynamics Question: Finding Fa using mass and acceleration

  1. Jul 21, 2012 #1
    Hello again forum :smile: I am still studying for that unit test and am still having problems with dynamics... I've attached the problem with the calculations/solutions. Basically, I don't understand why I'm getting the wrong answer. Probably b/c idk what the heck I'm doing :eek:

    1. The problem statement, all variables and given/known data

    15kg object thrown into the air. Initial acceleration is 8.8m/s^2. Find applied force.


    2. Relevant equations

    Please see attachment.

    3. The attempt at a solution

    Please see attachment. Btw, I realize Fa needs to be positive.

    Please advise regarding correct way of solving?.. Thank you for your time once again:smile:

    EDIT: I found my mistake. However, I still don't understand why "that" is the correct solution.
    My Fg value had to be negative;
    Fa= (15kg x 8.8m/s^2) - (-147.15)
    Fa= 279.15
     

    Attached Files:

  2. jcsd
  3. Jul 22, 2012 #2

    CWatters

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    Science Advisor
    Homework Helper

    The force available to accelerate the object (Fnet) is equal to the sum of all the forces acting on the object.

    Fnet = FA + (-Fg)
    or
    Fnet = FA - Fg

    Fg is negative because it's in the oposite direction to FA.

    Rearrange..

    FA = Fnet + Fg

    = ma + mg
    = m (a+g)
    = 15 (8.8 + 9.81)
    = 279 N
     
  4. Jul 22, 2012 #3
    You have done everything right, you simply made a mistake while solving the equations.
    You wrote:
    FA-Fg=ma
    FA=ma-Fg. <--- This is wrong. It should be FA=ma+Fg. :wink:
     
  5. Jul 22, 2012 #4

    CWatters

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    Well spotted. I looked at his working and didn't see that error.
     
  6. Jul 22, 2012 #5
    Thank you Watters :biggrin: That organizes it a little better lol. In regard to your last post, I am very much female by the way :wink:
     
  7. Jul 22, 2012 #6
    Eek! So sad that I didn't notice that... I'm sorry you had to point it out but thank you once again :biggrin:
     
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