Question 3
What must be done to find the resultant of two forces acting in parallel in the same direction?
The right answer is the one marked above.
They must be added. Activity 3 shows it: with the wooden strip at rest, the readings of A and B added together equal the reading of C.
This is the same as the first rule in section 9.2 — whether the forces lie on one line or on two parallel lines makes no difference to the sum. You subtract only when the forces oppose.