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Homework Help: Forces between charges. Did the problem, have question on the result.

  1. Apr 5, 2010 #1
    1. The problem statement, all variables and given/known data
    Three point charges are located at the corners of an equilateral triangle with side length a=0.50m

    2. Relevant equations

    3. The attempt at a solution
    My question is how can I subtract the vector F2/1? Vectors cant be negative right? I found the magnitude of the vector to be negative. Should I have not included the negative sign for the 4 micro coulomb charge? Then I add the two vectors and that will give me the correct result.

    My result is correct, but I cant minus vectors so I kind of did it illegally ya?

    Thanks for your help.
  2. jcsd
  3. Apr 5, 2010 #2
    Why do you think that vectors cant be negative? You can subtract vectors with no problem, whoever told you this was wrong. A negative vector is an arrow pointing in the opposite direction of whatever the unit vector is. basically the vector itself isnt negative, but the magnitude is negative. for example if I have a vector 3x (3 in the x direction) it represents an arrow of magnitude 3 pointing in the x direction. --->. A vector -3x (-3 in the x direction) just represents an arrow of magnitue -3 in the x direction. <---. Don't get confused by the term "vector addition". When we subtract, all we are really doing is adding a negative number. Subtracting vectors is solely adding vectors of negative magnitude. Which is totally fine.
  4. Apr 5, 2010 #3
    Ok, that sounds very familiar to how I studied it in math.

    I dont know why I can get confused with vectors.. I think its because my intro to physics teacher said that only magnitudes (scalars) are positive. If your vector comes up negative, you have initially set it in the wrong direction.

    I know part of that is true, or maybe all if it in a way, I just interpreted the first part wrong I think..

    So the work I wrote is correct? By adding the vectors, I subtract the components of F2/1 from F3/1
  5. Apr 6, 2010 #4
    Yeah it looks fine to me, i don't have time to look into detail. You should talk to your physics teacher and tell him that the way he is saying it is confusing. I am sure he will appreciate feedback on his teaching.
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