# Help with vector physics problem

1. Jan 24, 2008

### Yahaira.Reyes

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

Let vectors $$\vec{A}$$=(2,1,-4) $$\vec{B}$$=(-3, 0, 1), and $$\vec{C}$$= (-1, -1, 2) .
Calculate the following:

What is the angle$$\theta_{AB}$$ between $$\overline{A}$$ and $$\overline{B}$$ ?

2. Relevant equations

$$\vec{A}$$ * $$\vec{B}$$= magnitude of A * Magnitude of B * Cos$$\theta$$

magnitide of X= $$\sqrt{a^{2}+b^{2}+C^{2}}$$

3. The attempt at a solution

magnitude of A= 4.58
Magnitude of B= 3.16

$$\theta$$=arccos[( A * B)/ magnitude of A * magnitude of B)

2. Jan 24, 2008

### G01

I can't tell you if you did the arithmetic wrong if you don't show your calculations...

Also, it seems you posted this problem in the forum 3 times. Please do not flood the forum with repeat posts.

3. Jan 24, 2008

### Yahaira.Reyes

sorry if i posted it three times. It is my first time using this program.

magnitude of a= $$\sqrt{2^{2}+ 1^{2}+ 4^{2}}$$= 4.58

Magnitude of b= $$\sqrt{-3^{2}+ 1^{2}}$$= 3.16

and the vector A * the Vector B gives me -10

Last edited: Jan 24, 2008
4. Jan 24, 2008

### G01

OK, you calculations for the magnitudes are correct. Can you show me what you get for $$\theta$$ and the full calculation involved?

I ask because you have the correct formula, so I think the error is probably in your calculation somewhere.

5. Jan 24, 2008

### Yahaira.Reyes

a ha!!! i just saw my mistake!! But i will show you my calculations that i just re-did

$$\theta$$$$_{AB}$$= arccos[ -10/ (4.58* 3.16)]

= arccos(-10/ 14.47)
= 133.7

Thanks a lot!!!

6. Jan 24, 2008

;-)

I can't tell you the number of times I begin to spend the time typing up a problem here, and then all of a sudden I figure out the answer to my question.

7. Jan 24, 2008

### G01

Good Job!

8. Jan 25, 2008

### Yahaira.Reyes

i know i couldn't believe it! I was so frustrated trying the problem out over and over. I lauphed so hard when i figured it out lolz... I thought it was such a hard problem, turned out pretty easy