Is the reason the block slides down the plane that the resultant of the normal force N and mg acts along the plane causing it to slide or is it because of the component of mg parallel to the plane acting on the box i.e. mgsin(theta)
Oh, I see. Thanks!Right. They are in opposite directions and are equal. They add to a net of zero. All that is left after you add normal force plus gravity is the component of gravity parallel to the slope.
The resultant of normal force plus gravity is equal to mg sin theta.