# Homework Help: Chnage coordinate system

1. Aug 23, 2009

### LMZ

1. The problem statement, all variables and given/known data
2 coordinate systems are given:
1st: $$\vec{a}, \vec{b}, \vec{c}$$
2nd: $$\vec{m}, \vec{n}, \vec{p}$$
in system $$\vec{a}, \vec{b}, \vec{c}$$ basis vectors of 2nd system have values:
$$\vec{m}=\{2/3, 1/3, 1/3\}, \vec{n}=\{-1/3, 1/3, 1/3\}, \vec{p}=\{-1/3, -2/3, 1/3\}$$
also known that all 3 basis vectors of 2nd coordinate system have length 5 units and angle between each 2 vectors of 2nd system is 75 Grades.

2. Relevant equations

3. The attempt at a solution
1st equation: distance of vector $$\vec{m}$$ is $$|\vec{m}|=\sqrt{a^2*4/9 + b^2*1/9 + c^2*1/9}=5u$$
2nd equation: dot product $$(\vec{m}, \vec{n})$$ is $$|\vec{m}|*|\vec{n}|\cos{75^o}=25u*0.38=a^2*(-2/9) + b^2*1/9 + c^2*1/9}$$
next multiply 2nd with (-1) and substract from 1st 2nd:
$$a^2*2/3=5u(1-0.38); a \approx 2.15u$$, BUT length of the vector a should be greater then leght of the vector m, i guess...

Hope you'll understand what I mean ;)

Last edited: Aug 23, 2009
2. Aug 25, 2009

### kuruman

I don't think that the vectors in the first coordinate system are orthogonal. If they are, then the dot product of m and n would be zero which makes the angle between them 90o not 75o. Therefore, you cannot say that

$$|\vec{m}|=\sqrt{a^2*4/9 + b^2*1/9 + c^2*1/9}$$

You are missing the cross terms in the dot product

$$m ^{2} = \vec{m}\cdot \vec{m}$$

I assume you are looking for the magnitudes of a, b and c.