# Vector magnitude help

1. Jan 20, 2010

### matt72lsu

1. The problem statement, all variables and given/known data
Vector V1 is 6.4 units long and points along the negative x axis. Vector V2 is 8.9 units long and points at 60 degrees to the positive x axis
1) find x and y components of v1 and v2?
2) find magnitude of the sum v1+v2?
3) find the angle of the sum v1+v2?

2. Relevant equations

3. The attempt at a solution
I found the answers to part 1 (v1= -6.4,0 and v2= 4.5,7.7) and part 2 (8.0) but need help with part 3. thanks!

2. Jan 20, 2010

### rock.freak667

If you have |v1| ,v2| and |v1+v2|, then you have the 3 sides of a vector triangle.

You can get the angle between v1 and v2, since the angles at a point add up to 180°.

Then from there you can use sine or cosine rule.

3. Jan 20, 2010

### matt72lsu

im having trouble with v1+v2. i am getting -1.9 for the x component and 7.7 for the y. could i use tan-1 (y/x) = angle in this case?

Last edited: Jan 20, 2010
4. Jan 20, 2010

### rock.freak667

using that will give the acute angle measured clockwise. use tan-1(|y|/|x|) to get the magnitude of the angle and then 180- that angle to get it measured anti-clockwise

5. Jan 20, 2010

### matt72lsu

will it matter if i use a pos 1.9 instead of a -1.9?

6. Jan 20, 2010

### rock.freak667

you will get a negative angle is all.

7. Jan 20, 2010

### matt72lsu

so i got 76.1 degrees (using pos 1.9) so just 180-76.1= 104 degrees counterclockwise from the positive x axis?

8. Jan 20, 2010

### rock.freak667

Yes that should be correct.