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Math Help - More vectors

  1. #1
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    More vectors

    Given two vectors Vector A = 4.00i+7.00j and Vector B = 5.00i-2.00j , find the vector product A*B (expressed in unit vectors).

    Express your answers in i,j,k.

    I got 20.0i+14.0j+0k

    What is magnitude of the vector product?

    Either 280 or 0


    Please check it.


    Last edited by Oldspice1212; September 30th 2012 at 09:15 PM.
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  2. #2
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    Re: More vectors

    Hey Oldspice1212.

    Please state the problem precisely: I have absolutely no idea what you are trying to actually calculate since you just give an answer without a question.
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    Re: More vectors

    Well the first one it asked for Vectors A*B =which I have to put it in terms of unit vectors and second one is asking the same thing but now it's an absolute value so |A*B| for the magnitude
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    Re: More vectors

    Yeah but what is A and what is B?
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    Re: More vectors

    Oh I'm sorry I thought it was showing on my thread, I'll fix it right now

    There we go, let me know if it makes sense now.
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    Re: More vectors

    Quote Originally Posted by Oldspice1212 View Post
    Given two vectors Vector A = 4.00i+7.00j and Vector B = 5.00i-2.00j , find the vector product A*B (expressed in unit vectors).

    Express your answers in i,j,k.

    I got 20.0i+14.0j+0k

    What is magnitude of the vector product?

    Either 280 or 0


    Please check it.


    It looks like you have been trying to take the dot product, not the vector product, or cross product.

    The dot product results in a scalar, not a vector:

    A dot B = 4*5 + 7*(-2) = 20 - 14 = 6

    What method have you learned for calculating the cross product? Keep in mind the answer should be a vector that's perpendicular to both A and B.
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    Re: More vectors

    Ah I have no clue what the vector product is lol, I always thought you're suppose to use dot product for those kind of questions. What's the vector product?
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    Re: More vectors

    Quote Originally Posted by Oldspice1212 View Post
    Ah I have no clue what the vector product is lol, I always thought you're suppose to use dot product for those kind of questions. What's the vector product?
    The dot product is a scalar, a number, \vec{a}\cdot\vec{b} .

    The cross product, vctor product is a vector, \vec{a}\times\vec{b}.
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    Re: More vectors

    Didn't I do that? The answer should be the same since you're multiplying both right?
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    Re: More vectors

    Quote Originally Posted by Oldspice1212 View Post
    Didn't I do that? The answer should be the same since you're multiplying both right?
    There is no such an operation as vector multiplication.

    There are dot products and cross products but no vector multiplication.
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    Re: More vectors

    Ok so now I got 0i+0j - 43z...
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    Re: More vectors

    Is this correct since there are no z components not sure if it should be treated as 1 or 0
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    Re: More vectors

    Quote Originally Posted by Oldspice1212 View Post
    Is this correct since there are no z components not sure if it should be treated as 1 or 0
    If A=4i+7j~\&~B=5i-2j then A\times B=-43k
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    Re: More vectors

    Quote Originally Posted by Plato View Post
    If A=4i+7j~\&~B=5i-2j then A\times B=-43k

    Yes I understand that but I don't get the i and j should they be 0 or should I have put the z components as 1?
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    Re: More vectors

    Quote Originally Posted by Plato View Post
    There is no such an operation as vector multiplication.

    There are dot products and cross products but no vector multiplication.
    Actually Plato, the dot product is often called the scalar product, while the cross product is often called the vector product...

    Anyway, the easiest way to evaluate a vector product is using a determinant. If you have two vectors \displaystyle \begin{align*} A = a_i \mathbf{i} + a_j\mathbf{j} + a_k \mathbf{k} \end{align*} and \displaystyle \begin{align*}  B = b_i\mathbf{i} + b_j\mathbf{j} + b_k\mathbf{k}\end{align*}, then their vector product is

    \displaystyle \begin{align*} A \times B = \left| \begin{matrix} i & j & k \\ a_i & a_j & a_k \\ b_i & b_j & b_k \end{matrix}  \right| \end{align*}
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