# Thread: Vector Problems

1. ## Vector Problems

Hi
Need help on the following questions:
1)

Find the terms of a and b:
i)$\displaystyle \vec{OD}$
ii)$\displaystyle \vec{DC}$
iii)$\displaystyle \vec{DE}$

2)In the following case find the vector resolutes of the first vector parallel to and perpendicular to the second vector.
$\displaystyle a=2i-2j+k$
$\displaystyle b=i+j+4k$

P.S

2. Originally Posted by Paymemoney
Hi
Need help on the following questions:
1)

Find the terms of a and b:
i)$\displaystyle \vec{OD}$
ii)$\displaystyle \vec{DC}$
iii)$\displaystyle \vec{DE}$

2)In the following case find the vector resolutes of the first vector parallel to and perpendicular to the second vector.
$\displaystyle a=2i-2j+k$
$\displaystyle b=i+j+4k$

P.S
Have you tried anything? Is your trouble with the concept of vectors in their component forms? Or, is it with the concept of when vector are cosidered to be orthoganal? If you were to show some work or ask a question, I could help you.

3. ok well i tired the first two and i got $\displaystyle \vec{OD}=2a+3b$ and $\displaystyle \vec{DC}=2a+4b$.
I don't understand why this is incorrect, would you care to explain?

4. Originally Posted by Paymemoney
ok well i tired the first two and i got $\displaystyle \vec{OD}=2a+3b$ and $\displaystyle \vec{DC}=2a+4b$.
I don't understand why this is incorrect, would you care to explain?
Please show HOW you got these answers...

5. well i looked at the diagram and counted the squares for a and b.

oops i forgot to add that$\displaystyle \vec{OA}=a$and $\displaystyle \vec{OB}=b$

6. Well if you look at the grid, you'll see that 2 squares across = $\displaystyle \mathbf{b}$ and 1 square up = $\displaystyle \mathbf{a}$.

Since $\displaystyle OD$ is 3 squares across, wouldn't it mean $\displaystyle 1.5\mathbf{b}$? And then to go 2 squares up you would need to add $\displaystyle 2\mathbf{a}$?

So $\displaystyle OD = 2\mathbf{a} + 1.5\mathbf{b}$.

Now have a think about the others.

7. Originally Posted by Paymemoney
Hi
Need help on the following questions:
1)

Find the terms of a and b:
i)$\displaystyle \vec{OD}$
ii)$\displaystyle \vec{DC}$
iii)$\displaystyle \vec{DE}$
What are a and b? Do you mean the coordinate vectors i and j? Or do you mean OA and OB?

2)In the following case find the vector resolutes of the first vector parallel to and perpendicular to the second vector.
$\displaystyle a=2i-2j+k$
$\displaystyle b=i+j+4k$

P.S
I gave you the formula here:
http://www.mathhelpforum.com/math-he...resolutes.html

8. Originally Posted by HallsofIvy
What are a and b? Do you mean the coordinate vectors i and j? Or do you mean OA and OB?

I gave you the formula here:
http://www.mathhelpforum.com/math-he...resolutes.html
I mean OA and OB.

yeh, but i get the wrong answer:
This is what i have done:
$\displaystyle (2i-2j+k) \cdot (\frac{i-j-4k}{\sqrt6})(\frac{i-j-4k}{\sqrt6})$

$\displaystyle (\frac{2+2-4}{\sqrt6})(\frac{i-j-4k}{\sqrt6})$

9. Originally Posted by Paymemoney
I mean OA and OB.

yeh, but i get the wrong answer:
This is what i have done:
$\displaystyle (2i-2j+k) \cdot (\frac{i-j-4k}{\sqrt6})(\frac{i-j-4k}{\sqrt6})$
But you gave the second vector as i+j- 4k, not i- j- 4k

$\displaystyle (\frac{2+2-4}{\sqrt6})(\frac{i-j-4k}{\sqrt6})$

10. sorry about that, i made some typos.
Let me do it again, This is my full solution, according to the book's answers is incorrect.
$\displaystyle a=2i-2j+k$
$\displaystyle b=i+j+4k$

$\displaystyle =a \cdot \vec{b}(\vec{b})$
$\displaystyle =2i-2j+k \cdot (\frac{i+j+4k}{\sqrt6})(\frac{i+j+4k}{\sqrt6})$
$\displaystyle =\frac{4}{\sqrt6}(\frac{i+j+4k}{\sqrt6})$

$\displaystyle =\frac{4i}{6}+\frac{4j}{6}+\frac{16k}{6}$

$\displaystyle =\frac{4}{6}(i+j+4k)$

Now the answers says its $\displaystyle \frac{2}{9}(i+j+4k)$

I Hope this time i make sense.