The figure shows the free body diagram of the tube near the free end B. The load on top of the tube is P which equals 30 kilo newtons directed vertically downwards. The reaction force on member 2 is F subscript 2 directed vertically downwards. The reaction force at the base of the tube is F subscript 1 directed vertically downwards.

The force-deformation relationships

δ1=F1L1A1E1

and

δ2=F2L2A2E2

will be substituted into the geometry of deformations equation

δ1=δ2

to obtain the following equation:

F1L1A1E1=F2L2A2E2

This equation relating F1 and F2 is called the compatibility equation.