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Homework Help: Force Problem x-axis

  1. Apr 7, 2008 #1
    [SOLVED] Force Problem

    1. The problem statement, all variables and given/known data
    Only two forces act on an object (mass = 3.00 kg), as in the drawing (below). Find the magnitude and direction (relative to the x-axis) of the acceleration of the object.

    http://img148.imageshack.us/img148/6302/imgfbic2.png [Broken]

    2. Relevant equations

    F=ma

    3. The attempt at a solution

    F=ma
    40N=(3.00kg)(a)
    13.33m/s/s=a

    Correct answer: 30.9m/s/s, 27.2 degrees above the +x axis.
     
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Apr 7, 2008 #2
    there are two forces acting on it so you would have to find the resultant force because you have to find the force net in order to use the F=ma equation. Finding the resultant by using vector addition method will give you the Force net and then you can plug in the mass to solve.
     
  4. Apr 7, 2008 #3
    I understand what you're saying, but I'm confused. I thought I knew how to calculate the resultant vector, but the way I'm calculating it leads me to the wrong answer.

    Can you please show me how to do it? I would greatly appreciate it. ;)
     
  5. Apr 7, 2008 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Show how you calculated the resultant.
     
  6. Apr 7, 2008 #5
    Okay.

    I tried THREE ways:

    1. I averaged the two vectors: .5(60N+40N) = 50N... Then I divided by 3 (since the weight is 3.00kg and F=ma). That equaled 16 and 2/3 m/s/s. Wrong.

    2. I found the y component via Pythagorean theorem. It was √2000 = 44.7214N. Then I divided by 3 for the same reasons as #1. That equaled 14.97 m/s/s. Wrong.

    3. I did R = r- r0. (R being the resultant vector I am trying to find.). R = 40N. I divided it by 3 and got 13 and 2/3 m/s/s. Wrong.
     
  7. Apr 7, 2008 #6

    Doc Al

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    Staff: Mentor

    Why in the world would you (attempt to) take the average?

    In any case, do this: Find the x & y components of each vector. Add up the x components: that will be the x component of the resultant. Do the same with y components to find the y component of the resultant.

    You can then find the magnitude of the resultant via the Pythagorean theorem.
     
  8. Apr 7, 2008 #7
    you have to split the 60 N into two different vector quantities since it has a horizontal and vertical magnitude. the 40 N only has a horizontal quantity. then you would add the horizontal component of 60 and 40 to get the total force in the x direction.
     
  9. Apr 7, 2008 #8
    Haha, because none of the other ways I did it worked. :rofl:

    Thank you. I finally got the answer. :]

    Thank you for your help too. :]
     
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