# Geometric Vectors

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• Feb 17th 2008, 03:29 PM
Macleef
Geometric Vectors
I've been stuck on this question for over 2 hours now. . . I hope you can help me!

The homework question is:

Find the magnitude and the direction (to the nearest degree) of the resultant of each of the following systems of forces.

a. forces of 3 N and 8 N acting at an angle of 60º to each other - use geometric vectors to solve this problem

Answer: 9.8 N, 15º to 8N

I can't find the angle across 8 N or the resultant because I need to know the angle first before I can figure this value.

So my question is, "How do you find out the angle across 8 N in the following diagram?"

The only angle I know is 60º. I tried using the sine law, but I can't retrieve the angle to find the right resultant using the cosine law.

http://i28.tinypic.com/2e0psfa.png
• Feb 17th 2008, 04:09 PM
galactus
Maybe I am missing something, but it appears you either are lacking information or the problem/graph is stated incorrectly.
• Feb 17th 2008, 04:24 PM
Macleef
Nope, all the information is there and I don't think the problem is stated incorrectly
• Feb 17th 2008, 09:02 PM
topsquark
Quote:

Originally Posted by Macleef
I've been stuck on this question for over 2 hours now. . . I hope you can help me!

The homework question is:

Find the magnitude and the direction (to the nearest degree) of the resultant of each of the following systems of forces.

a. forces of 3 N and 8 N acting at an angle of 60º to each other - use geometric vectors to solve this problem

Answer: 9.8 N, 15º to 8N

I can't find the angle across 8 N or the resultant because I need to know the angle first before I can figure this value.

So my question is, "How do you find out the angle across 8 N in the following diagram?"

The only angle I know is 60º. I tried using the sine law, but I can't retrieve the angle to find the right resultant using the cosine law.

http://i28.tinypic.com/2e0psfa.png

You do have enough information.

Since all you have is the angle between the vectors, we may orient the vectors any way that we like, so long as there is still a 60 degree angle between them.

I'd simply set the 8 N force to act along the +x direction and then the 3 N force is acting at 60 degrees clockwise from this. Now add the vectors.

-Dan