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Math Help - vectors, find v, a, s

  1. #1
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    vectors, find v, a, s

    If both u X v = 0 and u.v = 0, what can you conclude about u or v
    i don't know what theyre getting at

    Find velocity v, acceleration a, and speed s at the indicated time t = t1 for r(t) = sin2ti + cos3tj + cos4tk; t1 = pi / 2
    im not sure where to start here, please help
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  2. #2
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    well u x v = 0

    you know |u x v | = |u||v|sin(b)

    b being the angle between the 2 vectors.

    So that means that b has to be 0 or 180 degrees.




    you know that u * v = |u||v|cos(c)

    where c is the angle between the 2 vectors.

    So if the dot product is equal to zero, then it means that the angle between the 2 vectors is 90 or 270.

    can you take it from there?

    then the second question you have

    v(t) is the first derivative of r(t), and a(t) is the second derivative of r(t)
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  3. #3
    MHF Contributor Amer's Avatar
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    Quote Originally Posted by Arez View Post
    i don't know what theyre getting at



    im not sure where to start here, please help

    it can be exist in the space two vector orthogonal and parallel to each other at the same time ??

    since if v\times u =0 we said that the two vectors are parallel and if v\cdot u = 0 the two vectors are orthogonal

    that can't be happened since if two vector are orthogonal they will intersect and they are parallel they will not intersect contradiction

    the second question

    velocity = r'(t)

    acceleration = r''(t) = v'(t)

    speed=\parallel v \parallel


    r(t) = \sin 2t i + \cos 3t j + \cos 4t k

    v(t)= 2\cos 2t i -3 \sin 3t j - 4\sin 4t k

    a(t) = -4\sin 2t i -9\cos 3t \hat{j} -16\cos 4t k

    s = \parallel v \parallel = \sqrt{(2\cos 2t )^2 + (-3\sin 3t)^2 + (-4\sin 4t)^2 }
    Last edited by Amer; September 8th 2009 at 10:33 PM.
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