# Magnitude of vector, negative in part.

1. Dec 10, 2011

### feathermoon

This is actually not a full problem, just a part of one I'm having trouble with:

1. The problem statement, all variables and given/known data

If I have a acceleration vector, say [ b$k^{2}$$e^{kt}$ - b$c^{2}$$e^{kt}$ ]$e_{r}$ + [ 2bkc$e^{kt}$ ] $e_{θ}$. How can I find its magnitude?

2. Relevant equations

Mag vector |a| = (a^2)^(1/2)

3. The attempt at a solution

As I square $_{e}r$, the cross term is still negative under the radical, and doesn't subtract cleanly:

[ $b^{2}k^{4}e^{kt}$ + $b^{2}c^{4}e^{kt}$ - 2$b^{2}k^{2}c^{2}e^{kt}$ ]$^{1/2}$

I'm either doing some wrong algebra or missing something obvious I think?

Last edited: Dec 10, 2011
2. Dec 10, 2011

### feathermoon

Well I'm stupid.. neglected e_theta completely out of sheer confusion from the first part... guess what it does when you add it in.

...Maybe this thread should be deleted. U_U Sometimes I just need a new perspective I guess.

3. Dec 10, 2011

### Redbelly98

Staff Emeritus

1. Note that $(e^{kt})^2 = e^{2kt}, \text{ not } e^{kt}$

2. You can enclose an entire equation or large expression in itex-/itex tags, you do not need to use separate itex-/itex tags for selected terms.

[ $bk^{2}$$e^{kt}$ - b$c^{2}$$e^{kt}$ ]$e_{r}$ + [ 2bkc$e^{kt}$ ] $e_{θ}$

you can write

$[ bk^{2}e^{kt} - bc^{2}e^{kt} ] e_{r} + [ 2bkce^{kt} ] e_{θ}$

which gives you

$[ bk^{2}e^{kt} - bc^{2}e^{kt} ] e_{r} + [ 2bkce^{kt} ] e_{θ}$

4. Dec 10, 2011

### feathermoon

Thanks Redbelly! I did have e^kt as e^2kt in my calculation, I just forgot to transcribe that correctly.

The itex thing on the other hand will probably save me some time on my next questions! :D