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Homework Help: Constrained to move Horizontally

  1. Aug 8, 2010 #1
    1. The problem statement, all variables and given/known data

    [PLAIN]http://img231.imageshack.us/img231/636/question2m.jpg [Broken]

    2. Relevant equations

    v = [tex]\dot{r}[/tex] [tex]\hat{r}[/tex] + r[tex]\dot{\vartheta}[/tex] [tex]\hat{\vartheta}[/tex]

    3. The attempt at a solution

    [tex]\dot{r}[/tex] = ?
    \vartheta = 80°
    v = 55mm/s

    So I guess I just use the formula above.

    v = [tex]\dot{r}[/tex] [tex]\hat{r}[/tex] + r[tex]\dot{\vartheta}[/tex] [tex]\hat{\vartheta}[/tex]

    55² = [tex]\dot{r}[/tex]² + rΘ'

    And so you try and solve for [tex]\dot{r}[/tex]

    r'² = 55² - (r*Θ')²
    r' = sqrt(55² - (r*Θ')²)

    And then I get stuck. I am either missing something. Or not doing something right. I guess this isn't really r theta, it is more a conversion from r theta to x-y.

    Not 100% sure how to do that though.

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Aug 8, 2010 #2


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    Homework Helper

    first you can write sevreal expresisons in tex as follows
    [tex]\vec{v} = \dot{r} \hat{r} + r \dot{\vartheta}\hat{\vartheta}[/tex]

    so knowing theta and |v| you should be able to decompose v into components in the orthogonal directions [itex]\hat{r}, \hat{\vartheta}[/itex]
  4. Aug 11, 2010 #3
    I manage to get the question.

    Thanks :D
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