# Help with physics:vectors

1. May 19, 2005

alright ive been stuck on these problems for the entire day. my teacher give us online homework, called mastering physics. the problem says to find the angle of each of the vectors.

A=Ax+Ay and B=Bx+By

they give me for for vector A, Ax=-1.40,Ay=4.2. now vector B, Bx=1.2,By=-2.6

so i pluged in, 108,204 and it comes back saying that my answer is off by an additive constant? what does that mean? ive tried looking at online tutorials but nothing explained what i did wrong. the answer is suppose to be in degrees. im getting really agitated

2. May 19, 2005

### dextercioby

Hint:compute

$$\vec{A}\cdot\vec{B}$$

Daniel.

3. May 19, 2005

ive tried the scalar product rule and still it doesnt work.

4. May 19, 2005

### Corneo

There are 2 equivalent ways of calculating a dot product.

$$\vec{A} \cdot \vec{B} = A B \cos \theta = A_x B_x + A_y B_y + A_z B_z$$

Where $$A$$ is simply the magnitude of the vector $$\vec A$$. Does that give you an idea now?

5. May 19, 2005

### whozum

I'm betting you forgot the negatives.

6. May 19, 2005