How to create Force output with expressions

Altair Community Member
Updated by Davide_21943
Hi,
I have a simple spring - mass system where a Force = 1 N is applied on the CM of the body.
I'd like to plot the stiffness, but I don't know how to do it.
I can't plot the force either. I create output, select expressions, in F2 I type the expression `FX({pmb_0.cm.idstring})`.
I select the CM marker of the body but the output is zero.
If I use the predefined Force output I can see that the force is 1 N, as it has to be.