A rod on two supports
Another situation with three parallel forces is a rod placed at rest on two supports.
Figure to be added
Here the weight of the rod and the two normal reactions the supports produce on it lie in one plane and are parallel to each other. Under these forces the rod rests in equilibrium on the two supports.
Different from the last case, and yet not
With the hanging bananas two forces acted upward and one downward. It is the same here — the two reactions up, the weight of the rod down. That is, one force acts opposite to the other two.
The pattern that keeps appearing with three parallel forces
If three forces are parallel, they cannot produce equilibrium while all acting the same way. One of them must always act opposite to the other two.
In the activity on the next page we measure the relationship between the magnitudes of those three forces.