It simply eliminates the need for a negative sign in the formula. Since this is a vector equation, the equation must give the correct direction for the force vector. This means a negative sign is needed. But swapping the indices from 21 to 12 is the same as a negative sign.Revered Members,
Kindly see my attachment.
Why for F\(_{21}\) the unit vector is written as r\(_{12}\) instead of r\(_{21}\)?
Yeah, these things are always confusing. You always have to carefully look at how the author is defining everything.Thanks for the reply steveb. Now look at this attachment. This material uses r\(_{21}\) for F\(_{21}\) which contradicts my previous attachment. Thats why i am confused.
Remember this is vector subtraction!accepted conventions that r21 is equal to r2-r1