Subtracting this vector - as opposed to adding it

[rpgnick85]

Homework Statement

Sketch the vector difference D = A - B

I have found F = A + B.

Homework Equations

A(x) = AcosTHETA
A(y) = AsinTHETA
B(x) = BcosTHETA
A(y) = BsinTHETA

R = sqrt(rx^2 + ry^2)

The Attempt at a Solution

As opposed to the F = A + B I have tried changing the signs of B or multiplying B(x) = -BcosTHETA etcetera and I can't come with the right solution. Do I need to put the angle 37 degrees in the second quadrant? I am confused as how to find -B as A is going to be the same as the first equation (F = A + B).

 

[SammyS]

Homework Statement

Sketch the vector difference D = A - B

I have found F = A + B.

Homework Equations

A(x) = AcosTHETA
A(y) = AsinTHETA
B(x) = BcosTHETA
B(y) = BsinTHETA

R = sqrt(rx^2 + ry^2)

The Attempt at a Solution

As opposed to the F = A + B I have tried changing the signs of B or multiplying B(x) = -BcosTHETA etcetera and I can't come with the right solution. Do I need to put the angle 37 degrees in the second quadrant? I am confused as how to find -B as A is going to be the same as the first equation (F = A + B).

Hello rpgnick85. Welcome to PF !

You would have

-B_x = -Bcosθ

-B_y = -Bsinθ​

That puts -B in the third Quadrant, doesn't it?

 

[rpgnick85]

-Bx = -Bcosθ

-By = -Bsinθ

Does that mean that I would do -18cos(217) for my first value?

 

[SammyS]

-Bx = -Bcosθ

-By = -Bsinθ

Does that mean that I would do -18cos(217) for my first value?

No.

The x component of -B is either 18cos(217°) or -18cos(37°) .

 

[rpgnick85]

alright so when i use -18cos37 and -18sin37 for my Bx and By values respectively and 12cos180 and 12sin180 for my Ax an Ay values I am coming up with R= 28.5126 and θ=22.33° which is incorrect

 

[rpgnick85]

found the answer- was in the wrong quadrant