Three-Force Member

3 forces in equilibrium are concurrent or parallel

A member with forces at only three points is a three-force body. Those lines of action meet at one point (or are parallel). Use this to find unknown directions before ΣF.

$F 1, F 2, F 3$: The three forces

Step-by-step solved example

A boom is pinned at A, cable at B, crate hanging at C. Where do force lines meet?

Three force lines concurrent (or parallel) for equilibrium.

1. Recognize
Boom has three forces: pin (unknown dir), cable (along cable), crate (vertical).
2. Concurrency
The pin reaction must pass through the intersection of the cable line and the vertical through C.

Answer: Pin reaction through the intersection of the cable and the load vertical

10 practice questions

0/10 correct

1.Two-force member forces are

2.If three forces are not concurrent and not parallel the body

3.A frame member with loads along its length is

4.Cables (no bending) align with

5.Parallel three-force case is

6.Pin reaction has

7.A pulley axle on a boom adds

8.To find the pin direction graphically

9.Roller support is

10.Internal pin of a two-bar frame