# Homework Help: Finding resultant force

1. Dec 18, 2007

1. The problem statement, all variables and given/known data
Okay, I am only allowed to use law of cosines and sones to find the resultant force F_r

I am having a hell of a time finding a usable angle after drawing my parellelogram..so obviously I am in need of sleep.

What am I missing here? (pic is clickable)

Casey

2. Dec 18, 2007

I know I need to isolate a triangle, but I really suck at similar triangles when they are not necessarily right triangles.

3. Dec 18, 2007

### Staff: Mentor

Draw yourself a diagram where you add the vectors (head to tail addition). The two vectors are the two sides of a triangle. You should be able to identify the angle between them and apply the law of cosines to find the third side of the triangle, which will be the resultant.

4. Dec 18, 2007

### G01

The 120 degree angle should be the angle corresponding to your resultant vector in the triangle. For the other angles:

HINT: What is the angle between the 80lb force and the -x axis? Does this help you find another angle in your parallelogram?

5. Dec 18, 2007

Is the angle between them by any chance 60 degrees? In the parellepgram 2(120)=240 leaving 120/2 to give me four angles that add to 360

6. Dec 18, 2007

### Staff: Mentor

Yes. (Assuming you're talking about the triangle I referred to.)

7. Dec 18, 2007

I'll be back in 20 minutes. All of the theological students just showed up at StarBucks and I can't take their banter.....I'm going home.

Casey

Angle between 60 and 80 with tail of 60 at tip of 80.

8. Dec 18, 2007

### Staff: Mentor

Run for it, man!

9. Dec 18, 2007

Thanks Doc and G01! I am taking this Statics course over X-mass break :points gun into mouth and fake blows brains out:

So is the general approach to these to use the fact that vectors can be moved around to redraw the scenario in a manner that helps to generate more information from the given info?

Casey

10. Dec 18, 2007

### Staff: Mentor

Yes. When adding vectors, you can move them around at will as long as you keep the magnitude and direction the same.