1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook