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How is a Newton a Unit of force, doesnt include direction

  1. May 16, 2013 #1
    If force includes strength/magnitude and direction, then why do we say a Newton or Pound is a unit of force?

    Isn't it more a unit of the magnitude of force? Please help. Now that i have been thinking about forces always having direction, this is all confusing me. Thanks
     
  2. jcsd
  3. May 16, 2013 #2

    CWatters

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    The Newton is a derived unit..

    http://en.wikipedia.org/wiki/Newton_(unit [Broken])

    So the units of force are really kgm/S2
     
    Last edited by a moderator: May 6, 2017
  4. May 16, 2013 #3
    But why is it still called a unit of force, shouldn't it include direction just as force has direction?

    What is confusing me is that Force has magnitude and direction, so where is the direction in the unit? Part of me would think in ordered to be called a unit of a force, it would have to have both as well?

    Kinda like a inch is a small unit of distance, mph is a small unit of speed. Therefore a newton should be a small portion of force...but force is not only the magnitude, that capacity to do the movement but the direction. THis is what is confusing me
     
    Last edited: May 16, 2013
  5. May 16, 2013 #4

    Doc Al

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    The Newton is the unit of force, but of course just the magnitude of force. Direction is specified in other ways, the same for any vector. To fully specify a force, you must give magnitude (with units) as well as direction.
     
  6. May 16, 2013 #5

    D H

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    Direction is unitless. Somewhat paradoxically, unit vectors are unitless.
     
  7. May 16, 2013 #6
    Doc Al, can you check my understanding?

    So what you are saying is that the unit of force, a newton is the unit used to quantify a force. A newton is the force required to move a 1kg object 1m/s^2. This just how it is.

    Really, to fully describe the force, it also has to have direction, but this is specified with its magnitude seperately. Force is fully a magnitude with direction. So for example the force of a car might be "500lbs ~northeast"

    But to repeat, the force still must include some unit, something that breakes it down. That would be the newton. I am probably over thinking this. tell me if this sounds right, thank you
     
  8. May 16, 2013 #7

    CWatters

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    How could direction be included in the units? There is no universal "zero direction", not least because the laws of physics appear to be the same in all directions. I suppose it would be possible to define an international "zero direction" from which all vectors were defined but that would certainly complicate more problems than it would solve.
     
  9. May 16, 2013 #8

    CWatters

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    Sounds fine to me. Usually the nature of the problem will suggest a suitable reference direction or the problem question may ask for an answer relative to some reference (eg Give your answer in degrees clockwise from north).

    In some cases it helps simplify a problem if you change your frame of reference, solve the problem, then convert it back to the reference requested in the question.
     
  10. May 16, 2013 #9
    Thanks all for the help. It is nice to have piece of mind and feel i understand this again.

    I guess what got me is that Force is truely understood as a magnitude and direction. But in so many problems, we are only concerned only with the general load/strength/magntitude of the force. Direction is simply left out or understood.

    For example, we might ask to find the force given a mass and an accleration, but direction is negilable. We end up with an answer like "force is 20 lbs". Part of me started to wonder, where is the direction?

    But then i realized it is somewhat taken for granted and really we are only concerned with the general force in the problem, hence the simplification. I guess i never had thought about that, and of course when i did it confused me. Thanks for helping me realize that force really is "magnitude and direction" but many times we talk simply about the magnitude of force as 'the force' too.
     
  11. May 16, 2013 #10

    Doc Al

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    Makes sense to me. There's nothing wrong with saying that a 1 N (net) force is required to give a 1 kg object an acceleration of 1 m/s^2. Note that no mention of direction is required, because maybe we just don't care. (Acceleration, to be fully specified, also requires a direction. It's just as much a vector as force is.)
     
  12. May 16, 2013 #11

    arildno

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    Sounds good!

    Forces can vary in their strengths (i.e, magnitude), as well as in direction.

    It is, as you say, rather common to denote the strength of a force as "force" as well. You'll get used to that.
     
  13. May 17, 2013 #12
    I was wondering why in my physics book it keeps talking about Forces as vectors, showing them as BOLD F, but then as the problem progresses, they simply use an unbold F.

    I now understand that strictly speaking, Force like acceleration is a vector, having both a magnitude and direction, but for simplicity(or the mere fact of not caring), we generally work on Force and acceleration more/less as a scalar, because the direction is just understood. Since most of my problems are fairly one-dimension(all in the x or y), we kinda ignore direction and just think about force as "so many LBS or Newtons etc". At least that is what my highschool book shows.

    Would you agree with this ARILDNO? I take it this is normal practice? So it is like you said, better to get use to it that sometimes when we talk about force, we are being more general, just wanting to know the magnitude/strenght?

    Thanks
     
  14. May 17, 2013 #13
    I guess that is one more question...why do we take vectors like force and acceleration and simplify them, treating them more scalars and throwing the direction out?

    My book is full of problems just like that where the direction is not even spoken off, instead you just assumed...example 'find the accleration given 5N applied to 5kg object"

    The answer is really just magnitude of the acceleration? So is the math. Is this just becuase you have to starrt small and learn the basics...alot easier to be general durnign a problem since we are in one dimension, we already know direction but just want the basic acceleration to answer the question? Just curious i guess
     
  15. May 17, 2013 #14

    russ_watters

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    As said, sometimes you just don't care. If you want to find how much torque your car engine needs to generate to give you a certain acceleration, does it matter which direction the acceleration is in?

    If you want to find the force needed to push a box across the floor against friction, does it matter which direction you push it? The force and the acceleration are in the same direction so you really don't need to do anything with them.

    Now if you are dealing with motion and forces that aren't aligned, it starts to matter more.
     
  16. May 18, 2013 #15
    FORCE is a VECTOR quantity. A vector quantity consist of three things basically:

    1. Magnitude which is just a POSITIVE number.
    2. Direction or sense of a vector: Unit vectors define that.
    3. Units: as you can see.

    So unit and direction are different constituents of a vector, then why do you even think that we need to mention the direction of vector while defining its unit ?
     
  17. May 20, 2013 #16
    I see this all the time now. Wish I would have picked up on it earlier
     
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