1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

3D Vectors and resultant force

  1. Nov 17, 2013 #1
    1. The problem statement, all variables and given/known data

    The vertices of a quadrilateral are A(1,2,-1) B(-3,2,-3) C(4,1,-4) and D(2,-1,3). At A forces of magnitude 4, 5 and 4 are newtons act along AB, AC, and AD respectively. Find the resultant force.

    Looking for more of a hint than an answer.

    Any help would be appreciated x
     
  2. jcsd
  3. Nov 17, 2013 #2
    The problem essentially gave you the direction of three forces, and their magnitudes. From these you could write the three forces as vectors, [itex]\vec{F_1}[/itex], and then add the vectors to give you the direction and magnitude of the resultant net force
     
  4. Nov 17, 2013 #3

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Skittlz! Welcome to PF! :wink:

    Start by writing out the three vectors …

    show us what you get. :smile:
     
  5. Nov 17, 2013 #4
    By adding I get AB = (-2,4,-4) AC = (5,3,-5) and AD = (3,1,2) would I then add these 3 vectors to get the direction and then use Pythagoras to find the magnitude?

    Edit: this is wrong.
     
    Last edited: Nov 17, 2013
  6. Nov 17, 2013 #5

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi Skittlz! :smile:
    No, for the vector joining A and B, you need to subtract, (not add) …

    the vector AB is OB - OA (where O is the origin), ie the coordinates of B minus the coordinates of A. :wink:
    Yes. :smile:

    (but you will stil need to use moments, to find the correct line of application, or at least any point on that line)
     
  7. Nov 17, 2013 #6
    I see, thanks!

    One thing I still don't understand. what am I supposed to do with the given forces?
     
  8. Nov 17, 2013 #7

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    eg the force along AB is of magnitude 4 …

    the vector AB (found by subtraction) tells you the direction, and you then have to multiply that by something (it isn't 4 ! :wink:) to get the actual force …

    show us what you get :smile:
     
  9. Nov 17, 2013 #8
    I know not to multiply by 4 as that just makes the vector larger i.e 4 times larger in this case. But 4N is the force across the vector which would mean |AB| = 4 right? So given that AB = (4,0,2) - (by subtracting) the magnitude would be srt20 right? but we are told that it is 4 so do we multiply by arbitrary values of x,y,z so that srt(4x^2+0y^2+2z^2) = 4 ? or is this wrong?

    I feel like I'm missing something quite important here but not sure what - vectors are a very big problem for me :(
     
  10. Nov 17, 2013 #9

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    HiSkittlz! :smile:

    Think in terms of unit vectors.
    Right :smile:

    So the unit vector along AB is (4/√20, 0 2/√20) …

    that's what you multiply by 4 ! :wink:
     
  11. Nov 17, 2013 #10
    Ah I see now, so what i do is:

    Find each unit vector then multiply by the force across it e.g.

    unit vector of AB = 4 x (4/√20, 0, 2/√20)

    Then do the same for the other two and then add the forces to find the resultant force.
     
  12. Nov 17, 2013 #11

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    yup! :biggrin:

    (and use moments to find the line of application)
     
  13. Nov 17, 2013 #12
    Thanks for the help :D
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: 3D Vectors and resultant force
  1. 3D Vector Forces (Replies: 1)

Loading...