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Homework Help: Three forces act on a moving object

  1. Aug 28, 2011 #1
    1. The problem statement, all variables and given/known data

    Three forces act on a moving object. One force has a magnitude of 80.0 N and is directed due north. Another force has a magnitude of 60.0 N and is directed due west. What must be the magnitude and direction of the third force, such that the object continues to move with a constant velocity?

    Force 1 = 80.0 N due north.
    Force 2 = 60.0 N due west.

    2. Relevant equations

    a^2 + b^2 = c^2
    Theta = tan^-1(fy/fx)

    3. The attempt at a solution

    First I started by plotting the problem on a cartesian plane. I then took my known quantities of 80.0 N due north (along the positive y axis) and 60.0 N due west (along the negative x axis) and used pythag to find the net force, the answer of which was 100 N.

    I then attempted to find the direction using Theta = tan^-1(fy/fx) = tan^-1(80/60) = 53.13.

    My question is, am I right in assuming that I must counteract the first two forces with the third to ensure a constant velocity? If so I would appreciate any help in this regard.
  2. jcsd
  3. Aug 28, 2011 #2


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    Careful with your direction. 53.13 degrees with respect to what? West of North? North of West? Otherwise, your assumption is correct.
  4. Aug 28, 2011 #3
    Thanks. I will be sure to always define that in future :smile:

    So the next step in my problem having confirmed my original assumption (thanks again) would be to determine the magnitude of the third force and it's direction:

    The net force I worked out to be 100 N and the direction to be 53.13 degrees North of West :smile:

    So the magnitude would have to be 100 N in the opposite direction being 53.13 degrees South of East?

    I appreciate all help as I have just begun studying again and I want to fully grasp each concept as I go along.

  5. Aug 28, 2011 #4


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