Two forces have the same magnitude f, what is the angle between the two vectors if their sum has a magnitude of (a) 2f? (b) sqrt of 2f (c) zero? sketch the three vectors in each case.

Ok let me help you with this:

!n case a) Line both up head-to-tail in a straight line... that's the only way to get a sum 2F. In case b) sqrt(2) is the length of the hypotenuse in a 45 degree triangle, so the vectors must be at 90 degrees to each other in the case of c) lined up head to tail, the only way to get 0 is if they point in opposite direction

Ibelieve that the correct answer is: c. along plate boundaries