# Vector forces

http://htmlimg2.scribdassets.com/cjr8y68tsmk4nsw/images/3-f2357829f8/000.jpg

it's the picture on the bottom

P= 15
Q= 25

I was wondering if this problem can be done using components of the force, instead of using the parallelogram law or the triangle rule. I'm a little confused on vectors and am having a hard time trying to find the angles of vectors in order to do the cosine law.

i tried to break them up into components and said px, py, and qy were negative while qx is positive since that is the way it looks in the photo, but i'm not coming up with the right answer.

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gneill
Mentor
http://htmlimg2.scribdassets.com/cjr8y68tsmk4nsw/images/3-f2357829f8/000.jpg

it's the picture on the bottom

P= 15
Q= 25

I was wondering if this problem can be done using components of the force, instead of using the parallelogram law or the triangle rule. I'm a little confused on vectors and am having a hard time trying to find the angles of vectors in order to do the cosine law.

i tried to break them up into components and said px, py, and qy were negative while qx is positive since that is the way it looks in the photo, but i'm not coming up with the right answer.
Sure you can do it by components. The "parallelogram law or the triangle rule" deal with components, only graphically.

Why don't you present your calculations here so we can have a look?

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Oh, I actually got in now, I was using the wrong angles for them...
I was just wondering if you could also clarify how to find the angles when using the parallelogram law or triangle rule.. I'm still a bit unclear about that. When I add the vectors together, I can't figure out how to find the angles. Is that all geometry/?

gneill
Mentor
Oh, I actually got in now, I was using the wrong angles for them...
I was just wondering if you could also clarify how to find the angles when using the parallelogram law or triangle rule.. I'm still a bit unclear about that. When I add the vectors together, I can't figure out how to find the angles. Is that all geometry/?
Yes, it's all geometry. Either sine or cosine rules, or carve the figure into right angle triangles and use Pythagoras (components!).

Thanks, for example, on this problem:

http://i32.photobucket.com/albums/d9/XXHoR0HoR0XX/physicsquestiontwo.jpg?t=1298243404 [Broken]

and the solution is this:

http://www.cramster.com/solution/solution/858898

how would I know that the angle is 96.9 degrees?

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gneill
Mentor
The angle that V2 makes with the horizontal is about 53.1 degrees (according to the little 3-4-5 triangle next to it) This can be found as arctan(4/3). It is given that V1 makes an angle of 30 degrees with the horizontal.

So the angle between V1 and V2 is 180 - (53.1 + 30) = 96.87, or to one decimal place, 96.9.

You'll have to look into how the angles in parallelograms relate to each other.

Thanks a lot for your help, I think i understand a bit better now.