Constrained to move Horizontally

[Sheen91]

Homework Statement

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

Homework Equations

v = \dot{r} \hat{r} + r\dot{\vartheta} \hat{\vartheta}

The Attempt at a Solution

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

So I guess I just use the formula above.

v = \dot{r} \hat{r} + r\dot{\vartheta} \hat{\vartheta}

55² = \dot{r}² + rΘ'

And so you try and solve for \dot{r}

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.

Cheers

 

[lanedance]

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

so knowing theta and |v| you should be able to decompose v into components in the orthogonal directions \hat{r}, \hat{\vartheta}

 

[Sheen91]

I manage to get the question.

Thanks :D