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Homework Help: Investigating Newtons law

  1. Mar 31, 2016 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    no equations required

    3. The attempt at a solution
    i already answered the question, but i just want to know if i wrote the right direction for the Fnet.
    is it [N 10 W] or [W 10 N]?


    this is how i wrote it:
    Fnet: 1165 N [N 10 degrees W]

  2. jcsd
  3. Mar 31, 2016 #2


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    As I explained in another thread, N 10 degrees W would mean 10 degrees W of North. Is that what it looks like in your diagram?
    Also, you don't show how you got 10 degrees. It is correct, to that many digits. Can you quote one more digit?
  4. Mar 31, 2016 #3
    ok this is how i got 10 degrees and the Fnet:

    Step #1: Use the cosine law to solve for the magnitude of the net force

    c2 = a2 + b2 – 2abcosC

    c2 = (800 N)2 + (400 N)2 – 2(800 N)(400 N)cos150 degrees

    c = 1165 N

    Step #2: Use the sine law to solve for the angle of the net force

    SinB/b = SinC/c

    SinB/400 N = Sin150 degrees/1165 N

    b = sin-1 (400 N x Sin150o/1165 N)

    b = 9.8 » 10 degrees

    To determine the angle for the net force, subtract 20 fromb, as this results in the angle needed to describe the direction of the net force.

    20 degrees – 10 degrees = 10 degrees

    Fnet = 1165 N [N 10 degrees W] it can also be looked at as 10.2 degrees, i just rounded 9.8 to 10 exactly.

    The net force on the log is 1165 N at an angle of 10 degrees west of north.
  5. Mar 31, 2016 #4
    in this case, the 800 N vector is further to the north and the resultant (Fnet) is further to the west.
    i described the angle between them by saying [N .... W]
    so is this correct?
  6. Mar 31, 2016 #5


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    Yes, that's fine.
    This is the identical mistake you are making on the other thread. When stating the direction of the resultant, we no longer care about any of the directions of the contributing forces. We must quote the direction relative to the standard compass points.
  7. Mar 31, 2016 #6
    ok so in this example taken from my lesson, the 1.0 N vector is directed East, and the Fnet is further north, to describe the direction, they said the Fnet was E....N
    can't this method be applied to my problem as well?
  8. Mar 31, 2016 #7
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