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Lesson: Chapter 12 — Equilibrium of Forces

12.3. Equilibrium under three parallel forces 1 of 5

Three forces side by side

Now we look at a system in equilibrium under three forces. A bunch of bananas hanging from a light horizontal rod is a good example of one.

Figure to be added

Figure 12.8 — a horizontal rod with a bunch of bananas hanging from it, together with the two ropes the rod itself hangs from.

Here the rod, the two ropes it hangs by and the rope the bananas hang from all lie in one and the same plane. The forces acting along the three ropes are also parallel to one another.

Coplanar

All three forces lie in a single plane.

Parallel

Their lines of action are parallel — they never meet at all.

This is an example of a system in equilibrium under three coplanar parallel forces.

Where is the weight of the rod?

Counting the weight of the rod there are four forces in this system. But a light rod means a rod small enough for its weight to be disregarded, so that force is ignored here.