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Resultant = zero

  1. Aug 12, 2016 #1
    1. The problem statement, all variables and given/known data
    Resultant of which of a following may be equal to zero?
    1)10 N, 10 N , 10N
    2) 10 N, 10 N , 25 N
    3) 10 N, 10 N , 35N
    4)None of these
    More than one option can be correct.

    2. Relevant equations
    Addition of vectors

    3. The attempt at a solution
    1) Resultant of these three forces can equal to zero if two of the forces make 120 degrees angle with each other and their resultant is opposite in direction with the third force.
    Answer at the back of textbook says option 1 is correct. I don't know how can we be so sure that other two options are wrong?
     
  2. jcsd
  3. Aug 12, 2016 #2
    Hint: If you have a single force, how much force in the opposite direction would be required to balance out (or counter) that single force?
     
  4. Aug 12, 2016 #3
    What if the question was this (really, the problem does boil down to this):
    Which of the options could be side lengths of a triangle?

    How would you figure this out?
     
  5. Aug 13, 2016 #4
    Triangle Inequality Theorem??
    I fail to understand how is my problem reduced to side lengths of triangle. Care to explain?
     
  6. Aug 13, 2016 #5

    gneill

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    Staff: Mentor

    How might you go about summing a set of force vectors graphically? If the resultant of the sum is to be zero, where must the tip of the "last" vector end up?
     
  7. Aug 13, 2016 #6
    Ding ding ding:smile:
     
  8. Aug 15, 2016 #7
    I was adding the vectors using parallelogram.
     
  9. Aug 15, 2016 #8

    cnh1995

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    Homework Helper

    Try the polygon method.
    download.png
     
  10. Aug 15, 2016 #9
    res.PNG
    I don't understand how A + B + R will be zero ? shouldn't it be 2R?
     
  11. Aug 15, 2016 #10
  12. Aug 15, 2016 #11

    cnh1995

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    A+B=R. R is the resultant of A and B.
     
  13. Aug 15, 2016 #12

    cnh1995

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    You have three vectors and their resultant is given to be 0. How would you draw this situation using a polygon?
     
  14. Aug 15, 2016 #13
    You want to do A+B-R, which will give you zero if A+B=R.
     
  15. Aug 15, 2016 #14
    But we want to add vectors why will there be negative sign??
     
  16. Aug 15, 2016 #15
    In the picture below, the top diagram shows the addition of three vectors (V1, V2, and V3) and the resultant vector (the sum of those three vectors). The lengths of the line segments represent the magnitude of the vector. The bottom diagram shows three vectors that, when added, produce a resultant vector of magnitude 0 (zero). (That is why there is no resultant vector R shown - because it is 0 magnitude.) So the question is, if you have three vectors of various magnitudes connected end-to-start (in other words, the end of one connected to the start of the next), and you can point each of those vectors in whatever direction you choose (so long as you keep them connected end-to-start), can there ever be a situation where you would NOT be able to connect the end of the third vector back to the start of the first vector - in other words a situation where it was impossible for 3 vectors to sum to 0?

    Vector sum pic crop.jpg
     
  17. Aug 15, 2016 #16
    The answer from @TomHart is great, but in short, A+B-R is the same as A+B+(-R), where the last vector's magnitude is R (and the problem only gives you the magnitude), but it's direction is negative.
     
  18. Aug 15, 2016 #17

    David Lewis

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    Gold Member

    If you travel along the vectors connected head to tail, and end up back where you started, then the vector sum is zero.
     
  19. Aug 15, 2016 #18
    Thank you so much for unambiguous explanations. If there are vectors a, b and c. Then a+b+c using the polygon law, we will get a quadrilateral with four sides as a, b, c and resultant r. If resultant r is 0, then and then only we will get a triangle after addition. Similarly if four vectors are given & their resultant is zero they will form quadrilateral with four sides otherwise quadrilateral with five sides, fifth side being the resultant. Right?
     
  20. Aug 15, 2016 #19

    cnh1995

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    With five sides, it becomes a pentagon.
     
  21. Aug 15, 2016 #20

    gneill

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    Staff: Mentor

    Only if all the vectors have the same magnitude can a regular figure (square, pentagon, hexagon,....) result. That's generally not the case. Further, when you go beyond three vectors there's no guarantee that the vector path won't cross itself or have concavities:

    upload_2016-8-15_23-36-35.png

    So irregular polygons can be formed (as on the right in the figure above) but so can objects that aren't polygons at all.

    The case of three vectors that form a closed loop will always result in a triangle.
     
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