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Math Help - Cross product of vectors.

  1. #1
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    Cross product of vectors.

    Here's the problem: "Given that v = 2i + 3j - k and w = i - j + 2k, find (v x w) x w."

    For some reason, I can't seem to get the right answer for this question.

    v x w = [2, 3, -1] x [1, -1, 2] = (6-1)i + (4-(-1))j + (-2-3)k = 5i + 5j -5k

    Then, (v x w) x w = [5, 5, -5] x [1, -1, 2] = (10-5)i + (10-(-5))j + (-5-5)k = 5i + 15j - 10k.

    Could someone tell me where I went wrong?
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  2. #2
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    Quote Originally Posted by Rumor View Post
    Here's the problem: "Given that v = 2i + 3j - k and w = i - j + 2k, find (v x w) x w."

    For some reason, I can't seem to get the right answer for this question.

    v x w = [2, 3, -1] x [1, -1, 2] = (6-1)i + (4-(-1))j + (-2-3)k = 5i + 5j -5k

    Then, (v x w) x w = [5, 5, -5] x [1, -1, 2] = (10-5)i + (10-(-5))j + (-5-5)k = 5i + 15j - 10k.

    Could someone tell me where I went wrong?
    Looks fine.
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  3. #3
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    Hm... Okay. Maybe I'm just misinterpreting the question, then. The exact wording of the problems tells me to "calculate the given quantity." Is that not what I did? Or is it looking for an actual number?
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  4. #4
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    Quote Originally Posted by Rumor View Post
    Hm... Okay. Maybe I'm just misinterpreting the question, then. The exact wording of the problems tells me to "calculate the given quantity." Is that not what I did? Or is it looking for an actual number?
    The cross product is a vector normal to the vectors. Should it be (v\times w)\cdot w\text{?}
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  5. #5
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    Quote Originally Posted by dwsmith View Post
    The cross product is a vector normal to the vectors. Should it be (v\times w)\cdot w\text{?}
    No, the notation is given as (v x w) x w.
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