# Vectors in a 3D plane

1. Sep 14, 2013

### andrewkg

OK so I am a bit confused. I am doing multiplication of vectors. I am a bit confused about the angles between two vectors. Lets say a(vect)=(3.0)i-(4.0)j; b(vect)=(2.0)j+(3.0)k in unit vector notation. Or generally how are angles between two vectors in 3d defined. Not just in terms of the dot or cross equ.

2. Sep 14, 2013

### voko

It is not clear what your confusion is about.

Is it about the definition of the angle between two vectors?

Or is it about finding the angle between two vectors given their components?

3. Sep 14, 2013

### SteamKing

Staff Emeritus
AFAIK, dot and cross products are how angles between vectors are determined, esp. in 3D. It's not like you are going to slap a protractor on them and read off the angle.

4. Sep 15, 2013

### Staff: Mentor

Conceptually, the angle between two vectors is what you get if you put them together tail to tail and use a protractor to measure the angle that this forms. Calculationally, if you have the components of both vectors as in your example, equate the two common formulas for the dot product and solve for the angle:

$$A_x B_x + A_y B_y + A_z B_z = |\vec A||\vec B| \cos \theta$$

5. Sep 15, 2013

### andrewkg

Never mind I misread something earlier. Making my question very illogical. Sorry. Thanks though.

Last edited: Sep 15, 2013
6. Sep 15, 2013

### Staff: Mentor

Conceptually, you put the two vectors together tail to tail as jtbell says above. The two vectors will lie in a single two-dimensional plane (which may be slanted/tilting); in that two-dimensional plane you can use a protractor to find the angle just as you would if you had started with vectors in only two dimensions.