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## Topic review (newest first)

irspow
2006-01-15 10:13:05

I'm sorry, I forgot about the F1 - F2 equation, but it really is no different than what I stated above.  My last post was F1 + F2.  The only difference in that calculation and the subtraction one is that you must use the negative components of F2.  All of the summation and equation still hold true.

F = √[(3cos90° - 5cos170°)² + (3sin90° - 5sin170°)²]

F = √[24.24615776 + 4.54439691], arctan(2.131759112/4.924038765)

F ≈ 5.37N, 23.41°

irspow
2006-01-15 09:58:30

You can break up as many vectors as you want to add or subtract and use the Pythagorean theorem.

∑F = √[(∑x)² + (∑y)²]

Specifically the resultant force is;

F = √[(∑x)² + (∑y)²], arctan(∑y/∑x)

I actually think that this method is easier because you can simply break all the forces into two component columns and add them up.  Once you do that you can just plug them in to the above equation.

For the original equations posted this is what would happen.

F1 = 3N, 90°  and F2 = 5N, 170°

F = √[(3cos90° + 5cos170°)² + (3sin90° + 5sin170°)²]

F = √[24.24615776 + 14.96328757], arctan(3.868240888/-4.924038765)

F ≈ 6.26N, -38.15°

mathsyperson
2006-01-15 09:02:57

Ricky's method is perfectly correct. A quicker, but harder, alternative is to make the two known forces into two sides of a triangle and then use trigonometric formulae to find the length and angle of the third side.

However, this only works if there are only two forces involved. Ricky's way will work for any number of forces.

Ricky
2006-01-15 08:51:00

First, we need to make these vectors.

The vectors x direction is cos(θ).  The y direction is sin(θ).

<x, y> = <cos(90°), sin(90°)>

However, this gives us a magntidue of 1.  We don't want 1, we want 3:

3 * <cos(90°), sin(90°)> = <3*cos(90°), 3*sin(90°)>

And that gives us it in vector form.

Now do the same for the other vector, and you'll have to vectors:

v1 = <x1, y1> and v2 = <x2, y2>

v1 + v2 = <x1+x2, y1+y2>
v1 - v2 = <x1-x2, y1-y2>

zara
2006-01-15 08:18:56

I'll write the directions and magnitudes again.......

F1= 3N at 90°

F2= 5N at 170°

Ricky
2006-01-13 12:04:28

f2=170°

You gave a direction, but not a magnitude.  You need both.

zara
2006-01-13 09:23:37