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Homework Help: Kinematics question

  1. May 31, 2008 #1
    1. The problem statement, all variables and given/known data

    In the attachment.

    2. Relevant equations

    3. The attempt at a solution

    First of all, sorry for the English, I'm from Israel, speak Hebrew here.
    Ok, I found v = (-w*R*sin(wt), w*R*cos(wt)), a = (-w^2 * R*cos(wt), -w^2*R*sin(wt)).
    I did tg to ay / ax and found the angle of a.
    The problem is when I do tg to v, I come out with tg = -ctg, how can I solve that?
    Hope u got it, 10x in advance.

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  3. May 31, 2008 #2


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    Welcome to PF!

    Shalom asi123! Welcome to PF! :smile:

    (btw, we'd say "I used tg = ay/ax", or "I divided ay by ax to find the tg of the angle of a." :smile:)

    Two ways to do it …

    One … draw a diagram! … that's often a good idea anyway …

    in this case, you can see that v is parallel to (-sinωt, cosωt), and a is parallel to (-cosωt, -sinωt), so from ordinary geometry the angle between them is … ?

    Two … as you say, the tangent for a is the same as minus the cotangent for v.

    Well, ctgθ = tg(what)?

    And -tgθ = tg(what)? :smile:

    (oh … this assumes that ω is constant … are you sure it is?)
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