Question

what will be the direction of the resultant vector if pbar is anti parallel to qbar, i,e., theta= 180 degrees

Answer 1

Their resultant is A+B in any case, by definition. If, however, their magnitudes (sizes) are a and b and the two are antiparallel, the magnitude of their resultant will be a-b if a>b and b-a if a>b.

This may sound deep, but it isn’t. All I am saying is that ‘resultant’ is another word for the sum of two forces, the result of their being applied jointly to an object. If you have two people exerting a 20lb and a 30lb force to pull a weight in the same direction, their resultant will be a 50lb force. But if they are pulling in opposite directions (this is what antiparallel means), the resultant will be a 10lb force in the direction of the stronger force