This question primarily involves finding the direction of forces in a truss.

Let's give a scenario in Method of Joints

One source I found says that in finding the direction of the source, it would be beneficial to assume everything is tension, if one attains a negative value, the force would simply be in compression.

However, wouldn't this be inaccurate in resolving forces using equilibrium equations (summation of forces along x =0 ; summation of forces along y=0) as they are sign-sensitive?

For example, a truss with several members are given. For the first member, it is assumed that everything is tension; however, it is found that one internal force is compression. With Newton's third law, the adjacent joint will also experience an internal force that is in compression, should I change its direction? Would that not affect summation of forces along x=0 and summation of forces y=0 as mentioned?

Please enlighten me, thank you


It doesn't matter if you "guess right" whether each member is in tension and compression. The important thing is that the forces at the ends of each member are equal and opposite, and write all the equilibrium equations consistent with those assumptions.

If you guess the member is in tension, and you find the force is negative, you have two options:

  1. Don't change the direction of the forces on the diagram, and put the negative number into the equilibrium equation for the other end of the member.
  2. Redraw the diagram with both forces in the opposite direction, and rewrite the two equilibrium equations, so the magnitude of both forces are positive.

Option 1 is usually quicker and neater, since you don't have to keep redrawing the diagram and changing the equations you have already worked out.

Most computer software for analyzing trusses uses the convention that positive force in the members means tension and negative force means compression, so you might as well get used to using that convention when you solve problems by hand.

In practi

  • 1
    $\begingroup$ +1 for clarifying the options so nice. It seems you haven't your final sentence though. $\endgroup$ – NMech Jan 10 at 22:49

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