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- "Gentlemen, I give you the Long Fall Boot. Think of it as foot-based suit of armor for the Portal Device. I'm not gonna lie to you, it's expensive as hell. But check this out: we told this Test Subject to just go ahead and try to land on her head. Heh heh! She can't do it! Good work, boots."
- ―Cave Johnson[src]
The Long Fall Boot is a boot used by Aperture Science to prevent Test Subjects from inadvertently damaging the Portal Device while falling to their death. It does this by lessening the impact of falls, thus also preventing injury to test subjects.
During the testing of the Portal Device, the observers came across a reproducible human error - due to the conservation of the momentum of an object (or human) traveling through a portal, Test Subjects would reach great heights, only to fall and die on impact with the Test Chamber Floor, most likely due to damage to internal organs or because of massive blood loss. As Test Subjects were prone to flail uncontrollably when faced with their imminent demise, they would often drop the Portal Device, which would be damaged upon hitting the ground. The equipment was expensive and was costing Aperture Science time, money and manpower, so Cave Johnson devised the Long Fall Boot as a solution to the problem of falls from height leading to broken equipment, by way of canceling out the momentum conserved between portals. The Long Fall Boot is a new iteration of Aperture's Advanced Knee Replacement for Portal 2.
- The Long Fall Boot may be installed with a gyroscopic mechanism of some kind, as the boots aid the test subject in balance and keep them from landing on their heads when falling through or exiting portals. This feature is mentioned by Cave Johnson and demonstrated by Chell in Aperture Investment Opportunity #4: "Boots".
- When a test subject wearing the boots falls from a height that would require the boots to kick in, lest the equipment be damaged, they will make a sound similar to hydraulics moving.
- They are capable of surviving truly long falls, up to and possibly beyond the subject's terminal velocity, as it appears when Chell falls four kilometers down to old Aperture Science. However, this might not be true considering her being somewhat slowed by a layer of planks partially blocking the shaft she was falling down. They still survived the extremely long fall down to the Incinerator Room, which did not feature any blockages (although it did feature some non-vertical sections which may have slowed Chell's fall).
- It's possible that one boot alone can still protect a Test Subject from damage from a fall, as long as they land on the foot on which they are wearing the boot. This is mentioned by GLaDOS when she unsuccessfully attempts to get Chell to give her a boot to protect herself, saying "Just remember to land on one foot". However, it's possible that GLaDOS was concerned only for her own safety, and simply didn't know or care whether one boot would be enough to protect Chell.