What parallel forces must satisfy
For a body to be in equilibrium under three parallel forces, three requirements must be satisfied.
(i) The three forces must be coplanar
That is, all three must lie in one plane. This is what the last part of activity 3 demonstrated.
(ii) One force must oppose the other two
If all three act the same way there is no way for them to cancel.
(iii) The resultant of two must equal the third
Equal in magnitude and opposite in direction. That is, F1 + F2 = W.
Another way to read the third condition
When two parallel forces act in the same direction their resultant is simply their sum. So the third condition can be read directly as this: the sum of the two forces acting one way must equal the single force acting the other way.
Equilibrium under three parallel forces
A body is in equilibrium under three parallel forces when a force equal to the resultant of two of them acts in the opposite direction. All three must lie in one plane as well.