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Negative sign or not in calculations?

  1. Sep 27, 2015 #1
    Hi , this may been asked quite a bit times but I can't really find a solid answer for this.
    Do we have to include negative signs during calculations for vectors or scalars?

    I have been told by someone that for vector quantity calculations such as Force we do not include the negative sign or positive as we have to indicate the direction in a separate sentence however for Scalar we will have to indicate it as it does not have a direction

    However I have also been told by someone else that it is actually the other way round where we include negative signs for Vectors and not for Scalars.

    So can someone please clarify this with me?
    Thanks a bunch
     
  2. jcsd
  3. Sep 27, 2015 #2

    mfb

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    What do you mean with "include negative signs"?

    Both vectors and scalars can have signs, those can be positive or negative.
     
  4. Sep 27, 2015 #3
    For example in a calculation for electric force and electric field strength I was told to omit the sign of the charges while for electric potential and electric potential energy I was told to include it in.
    Also for F=ma , if my acceleration was a negative one (deceleration) I was also told to omit it from my calculation for force.
     
  5. Sep 27, 2015 #4

    mfb

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    In general, keep signs. If you know exactly which direction some force has and if you don't care about the other direction, it can be possible to ignore its sign, but always at your own risk then.
     
  6. Sep 28, 2015 #5

    CWatters

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    You don't need the sign if you are only interested in the magnitude of the force but in general you will need the sign to work out the direction of the force. It may come down to the wording of the question. Best keep the sign during calculations then think about what it means before giving a final answer.
     
  7. Sep 28, 2015 #6
    It depends on the meaning that you give to the symbols you write: if with "F" (for example) you mean "vector force" or "i-th component of the vector force", then you don't have to give it a sign at the beginning, because the sign is "contained" in the symbol itself.

    But if instead with "F" you mean "modulus of the vector force" or "modulus of the i-th component of the force", then you have to know in advance which is the direction of the vector force or of that i-th component and you have to give the proper sign to F.

    Example: if with F you mean ||Fx|| and positive x is what you have choosen for positive values of components, you have to write -F if you already know (or you intend to consider) that the vector force is directed as the negative x.

    For this reason in physics' problems it's very important to start having a very precise idea of the meaning you intend to give to the symbols you write.

    --
    lightarrow
     
  8. Sep 28, 2015 #7
    Thanks for the responses. What about charge? Do I include the negative sign of -1.60x10^-19 when calculating charge or just 1.60x10^-19?
     
  9. Sep 28, 2015 #8

    mfb

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    Include it.
     
  10. Sep 28, 2015 #9
    But if I do include it and say I get a -6000C, and the question then asks me to calculate the energy transmitted , and if I use the formula V=E/Q, wouldn't I end up getting a negative energy?
     
  11. Sep 28, 2015 #10

    mfb

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    Energy transmitted from what to what?
    Usually the problem statement asks for a direction where the energy change is positive, or asks for the magnitude of the energy change. Anyway, if your charge is negative then things will have the opposite sign compared to a positive charge, you cannot ignore that.
     
  12. Sep 28, 2015 #11
    Alright thanks alot once again!
     
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