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Thread: Homework assistance(Vectors) - no answers please.

  1. #1 Homework assistance(Vectors) - no answers please. 
    New Member
    Join Date
    Nov 2010
    We recently had an assignment in my Physics class. It's nothing entirely difficult for most of the members here but I thought I had done things correctly with the assignment.

    Turns out I hadn't. Could someone just show me how I might go about answering the following questions without providing me the answers? What steps would I take to answer each one because I fear I'm completely lost and do not want to fall behind.

    1.) Find the angles between the Vectors.

    A = 3i + 2j
    B = -j + 6j
    C = 7i - 2j

    2.) Given the two vectors A = -2i + 3j + 4k and B = 3i + j - 3k, do the following
    a) Find the magnitude of each vector
    b)Write an expression for the vector difference A-B using unit vectors
    c)Find the magnitude of the difference A-B. Is this the same as B-A? Explain.

    3.) Obtain a unit vector perpendicular to the two vectors given in problem #2.

    4.) Find the Cross product of A = i -2j + 3k and B = 2i +4j +2k

    5.) You are given vectors A = 5i - 6.5j and B = -3.5i + 7j. A third, C, lies in the xy plane. C is perpendicular to A,
    and the scalar product of C with B is 15. Find the components of C.

    6.) Two vectors, A and B have the same magnitude, A = 3 and B = 3. Their vector product is AxB = -5k +2i. What
    is the angle between them?

    Any amount of help with any of these is appreciated.

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  3. #2  
    Forum Professor wallaby's Avatar
    Join Date
    Jul 2005
    Seeing as you're only looking for hints i'll have a go at providing some, although i'm not sure how the mods will feel since i think the community usually likes to see at least one attempt at any questions asked that come from books, assignments, exams (past, present or future)

    Looking at all of these questions it's fairly obvious (to me) that you're being examined on the inner (dot) product and the cross product. The magnitude of a vector and the angle between two vectors are pretty much defined by the inner product, so as long as you remember what the inner product is and how you use it to find length and angles you should able to answer at least half the questions there. For the other half you need to recall the properties of the inner and cross products. For instance, the cross product of two vectors gives a third vector that is perpendicular to the first two. Also, two vectors that are mutually perpendicular will have an inner product of zero.

    Hopefully my hints haven't been too obvious, or too subtle.

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  4. #3  
    Forum Bachelors Degree
    Join Date
    May 2010
    A vector is simply a magnitude with a direction.
    If it makes it more simple consider the i and j perpendicular directions on a cartesian plane ( x-y graph paper ), you then have two line segmants and I assume you know how to find the angle between two line segments. the magnitude is simply the length os the line segment and I assume you can calculate that also.This can be extended to three dimensions as well.
    If all else fails you can graph the line segments and actually measure the angles and lengths although I doubt you'll get marks for this. On the other hand it may help you understand vectors a little better.
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