# Homework Help: Vector Question - Can this be solved?

1. Sep 19, 2009

### Bymp

I just had a test Friday and I spent 30 minutes on this so-called "Easy" problem. The resultant force is "-2J + 5K" and there are 3 forces that compose this resultant force.

The Magnitude of F2 is 5kN
The Magnitude of F1 is 3kN
F1 has a angle of "0" to the Y axis

The 3 forces are all acting on a 3D vector plane, but no vector has a value for "I". So none are moving in the x direction.

Given the image below (Forgive the crude drawing):
Find:
The Magnitude of F3
The Angle that F3 is acting apon
And the Magnitude of the Resultant force.

Can this be done given the information in my drawing? I didn't leave anything off, this is what we were given and I even asked my instructor if we were missing information. I couldn't solve this one, and many in my class had problem just as I did. This is a second year college level course.

If you can help me out, thanks, I really would love any help I could get.

Last edited: Sep 19, 2009
2. Sep 19, 2009

### Staff: Mentor

No angle given?
I presume you mean 0 degrees to the y-axis.

3. Sep 19, 2009

### Bymp

Actually I didn't understand that part either, what he place on the test looked like a letter "O" for the angle, so it confused me. I persumed that I could solve the angle there by using the formula:

Cos Angle = (A *dot* B) / [(Magnitude of A) * (Magnitude of B)]

I believe that's a common vector reference. I believe I typed that correctly

4. Sep 19, 2009

### Bymp

I fixed that as well, the angle was 0 degrees to the x- axis, thank you

5. Sep 19, 2009

### Staff: Mentor

I don't understand. Are you given the scalar product of A and B? (Sure, if you're given the magnitude of two vectors and their dot product, you can get the angle between them. But are you given that information?)

Was the direction of F2 specified or just its magnitude?

6. Sep 19, 2009

### Bymp

Oh, true, I was mistaken, your right. I apoligize.

All information supplied in the image and the first post should be 100% correct now.

What is supplied what what was supplied.

I'm just trying to think of ways I could have solved this. He claimed this was an "Easy" problem requiring only 10 minutes of work at the most.

But you are right, we can't use that because we are not given the scalar product of the two.

7. Sep 19, 2009

### Bymp

I just can't find any way to solve for the wanted values =\

Sept the magnitude of the resultant force.... that's the only way I could solve for.

8. Sep 19, 2009

### Staff: Mentor

Sure looks like insufficient information to me.

9. Sep 19, 2009

### Bymp

Thanks =) Just glad someone agrees with me, thank you very much, my mind is at better rest now