M1 - resultant forces - angles question

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?

Reply 1

That bloke 2016

1

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.

Reply 2

That bloke 2016

1

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

(edited 7 years ago)

Reply 3

Sensei_Stig

1

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?

Reply 4

That bloke 2016

1

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?