# Dot product

student 1

## Homework Statement

A. If A*B=-(1/2)AB, what do you know about A and B.
B. If A*B=5Ay-7Ax, find B.

## The Attempt at a Solution

Part A I know the angle is 120 degrees in between A and B. Would I know more about the two vectors from that?
Part B. I don't know where to begin to try and answer that?

## Answers and Replies

neutrino
For A, it could be also be 240 degrees.

For B, think in terms of the components of the vectors. How do you represent a scalar/dot product of two vectors in terms of their components?

student 1
AxBx+AyBy with that I should be able to find my B?

neutrino
Yes.

student 1
Do I use what I know in the first part of the problem as well or would this be a totally different scenario??

neutrino
Do I use what I know in the first part of the problem as well or would this be a totally different scenario??
A better question that you must ask yourself should be, does the the answer to the first problem help you in anyway in solving the second, given that you know component-wise multiplication of vectors?

student 1
So Bx and By would simply be 1 since they are not in the problem right?????

physixguru
Of course not.You have to solve in the context of the upper question.

student 1
According to the teacher part A and B are unrelated, they are two separate problems, in that case bx and by would be 1 correct?

Homework Helper
Part B tells you that A*B=5Ay-7Ax. And, of course, A*B= AxBx+ AyBy. Just compare those two equations!

Hi student 1! (have you noticed the new X2 and X2 tags? )
Hint: If 5Ay - 7Ax = A.C also, what can you say about B - C? 