# Combining forces.

1. Sep 10, 2011

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

3. Draw figures showing how you would combine a 6 N force and an 8 N force to obtain a resultant force with magnitude (a) 14 N; (b) 2 N; (c) 10 N.

2. Relevant equations

3. The attempt at a solution

I just don't know where to start or what equation to use.

2. Sep 10, 2011

### cepheid

Staff Emeritus
Do you understand how to calculate a vector sum graphically? That's all this question is really about...

3. Sep 10, 2011

In our lab we graphed forces, but they had an angle as well.

4. Sep 10, 2011

Oh wait, like draw a straight line with 8 and 6 for 14, while drawing lines with arrows towards each other for 2?

14
-------------->------------------------->

2
--------------><-------------------------

5. Sep 10, 2011

### cepheid

Staff Emeritus
Well, you've found two combinations that work. If the forces are in the same direction, the magnitude of the resultant is just the sum of the magnitudes of the two vectors. Likewise if they're in the exact opposite direction, the magnitude of the resultant is just the difference of their magnitudes. But the more general case is when they are not co-linear, meaning along the same line. I.e. the angle between them is something other than 0 or 180 degrees. How do you find the resultant in this case? In other words, how do vectors sum together? You should know this.

6. Sep 10, 2011

Well, I can add vectors(of force) by finding components, graphing them, or using a force table.

I don't really know how to add vectors that well, but wouldnt there be a 90 degree angle between 6 and 8 to give me 10, since 8^2+6^2 square root= 10.

Thanks.

Last edited: Sep 11, 2011
7. Sep 11, 2011

### cepheid

Staff Emeritus
Yeah. So you figured out the answer to all three cases by trial and error or "guess and check." But the general method for finding the resultant is either to resolve each vector into components and add them component-wise, or to do it graphically by using the "parallelogram rule" for vector sums (which I can only assume you must have learned):