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M1 - resultant forces - angles question Watch

1. Two forces of magnitude 6N and 5N act on a particle. The angle between the forces is 40 degrees. Find the magnitude and the direction of the resultant force.

I know you have to use the cosine rule, and in my workinh out I did a^2=b^2+c^2-2bcCosA where cosA is cos40, but the book has cos140 instead. How did they get this?
2. Not too sure if this is right but whenever these Q's come up, the angle I always use is 180 - the stated angle or in this case 140.
I do it because it always forms the correct triangle.
3. (Original post by _Tyrion)
Two forces of magnitude 6N and 5N act on a particle. The angle between the forces is 40 degrees. Find the magnitude and the direction of the resultant force.

I know you have to use the cosine rule, and in my workinh out I did a^2=b^2+c^2-2bcCosA where cosA is cos40, but the book has cos140 instead. How did they get this?
Getting an answer of 2 degrees for the direction...

EDIT: 21.86 degrees. I miscalculated
4. Is it because you didn't resolve the forces? Remember you have to form a triangle with the forces placed 'nose-to-tail' before you can solve properly. This might make the difference?
5. (Original post by Sensei_Stig)
Is it because you didn't resolve the forces? Remember you have to form a triangle with the forces placed 'nose-to-tail' before you can solve properly. This might make the difference?
Ah good someone knows the official term for my 180 - angle trick.
6. You're welcome! :')

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