- The overview of physics problems in drilling is overly simplified. There is a reason it takes years to become a drilling engineer.
- Not taken into account is buoyancy. Most of the hole being drilled is filled with liquid, the mud. The is going to be placing an upward force on the whole drillstring.
- Another force is friction. There is going to friction in both the plane of circular motion, and in the vertical plane. The friction comes from mostly the solid particles of rock flowing back up the hole with the mud.
- The pipe has some elasticity to it. To say not to allow force on the pipe in the vertical direction is if it were in perfect conditions. Since nothing in the world is perfect, the pipe is rated to allow some force on the drillstring.