"Line of action of a force"

One of the problems from the Feynman Lectures asks us to show that three
forces in static equilibrium must all be coplanar, and their lines of
action must pass through a single point.

If someone were to ask me what is meant by "line of action of a force", I
would probably say the line parallel to the force and passing through the
point of application. By that definition, the proposition of the problem
is wrong.

Here's someone else's definition:
http://www.arch.virginia.edu/~km6e/references/glossary/struc-glossary.html#L_glossary

"Line of Action: The line of action of a force is the infinite line defined
by extending along the direction of the force from the point where the
force acts."

Take a long narrow board and apply three forces to it. let the forces be in
the same plane, and let two of the forces act in the same direction
perpendicular to the long axis of the board. Apply a third force in the
center of the board in the opposite direction. That will achieve static
equilibrium, and not all of the lines of action pass through the same
point.

Is there another definition of "line of action of a force"?
--
http://www.vho.org/GB/c/DC/gcgvcole.html
http://www.vho.org/GB/Books/dth/
http://www.germarrudolf.com/
http://www.ice.gov/pi/news/newsreleases/articles/051115chicago.htm
.

Relevant Pages

  • Re: "Line of action of a force"
    ... The line of action of a force is the infinite line defined ... and let two of the forces act in the same direction ... It's the limiting case as the point of intersection of the lines of action (PILA) recedes to infinity. ...
    (sci.physics)
  • Re: "Line of action of a force"
    ... would probably say the line parallel to the force and passing through the ... and let two of the forces act in the same direction ... The farther away the point in common, ...
    (sci.physics)