What is the correct method for finding the angle of vectors in physics?

[badman]

alright I've 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? I've tried looking at online tutorials but nothing explained what i did wrong. the answer is suppose to be in degrees. I am getting really agitated

 

[dextercioby]

Hint:compute

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

Daniel.

 

[badman]

ive tried the scalar product rule and still it doesn't work.

 

[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?

 

[whozum]

I'm betting you forgot the negatives.

 

[badman]

lmao i did forget the negative. :( but thanks a lot for your help guys. and this is a very cool site