# Adding Vectors

1. Sep 12, 2009

### devmgnt

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

If B is added to C = 3.0î + 4.0ĵ, the result is a vector in the positive direction of the y axis, with a magnitude equal to that of C. What is the magnitude of B?

2. Relevant equations

a = sqrt(ax2 + ay2)

3. The attempt at a solution

1. Found the magnitude of C

c = sqrt(cx2 + cy2) = sqrt(32 + 42) = 5

2. Added B to C and got this equation

5 = sqrt(ax2 + ay2) = sqrt(b2 + 52)

which obviously isn't right because I get the magnitude of B as 0. I'm not sure what I am doing wrong, but any guidance would be helpful. Thank you.

2. Sep 12, 2009

### CFDFEAGURU

Think it through.

The equation you are trying to solve is;

B+C=C

In words "the magnitude of B plus the magnitude of C equals the magnitude of C"

You don't even need to find their magnitudes.

Make sense yet?

Thanks
Matt

3. Sep 12, 2009

### mikelepore

If B + C = result R, then B = R - C. Convert the phrase "the result is a vector in the positive direction of the y axis, with a magnitude equal to that of C" into a components and a unit vector, and then subtract C from it, vector arithmetic. They're asking you for the magnitude of that last one.

4. Sep 12, 2009

### mikelepore

That's not what the question says.

5. Sep 12, 2009

### CFDFEAGURU

"That's not what the question says."

No, I don't agree.

Thanks
Matt

6. Sep 12, 2009

### mikelepore

C and "the result" R have the same magnitudes, and yet C has two positive components, while R is entirely in the +y direction. What are you gonna add to C to get R? Hint: Its magnitude will be an irrational number that has to be rounded off according to the number of significant digits in the given components of C.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook