What Is a Gravity Knife?
A gravity knife was developed prior to the creation of a reliable and cheap switchblade knife. It was invented in World War II and issued to paratroopers in case they had to cut themselves out of their parachutes should they become tangled in trees or other objects upon landing. A gravity knife relied on inertia and the force of gravity to open, as spring-driven knives were not popular at the time. The allure of such a knife, compared to a simple folding knife, was that it could be opened and used one-handed. The closing mechanism of a typical folding knife is often too stiff to be flipped open one-handed. This was a compromise from the standard fixed hilt combat knife that standard infantry carried. While the fixed hilt combat knife could indeed be drawn with one hand, it was bulky and could become dislodged from its sheath during parachute drops, while a gravity knife could be safely secured in an inner pocket with a zipper.
How Does a Gravity Knife Open and Close?
There are two types of gravity knives, each opens differently. The standard type looks no different from a folding knife, with a hinge on the side. To open this model, a lever on the side was pressed down. This lever was what secured the blade and kept it from twisting loosely on its hinge. With this done, all the user needed to do was flick his wrist in a back-handed motion. The inertia of the flick pulled the blade out entirely, while releasing the lever locked the blade in the open position. The second type of knife was an OTF, or Out the Front design, where the blade was retracted in the hilt and designed to slide out straightforward. Again, a lever was in place, this time over the front of the slot at the top of the hilt. The lever was pulled out of the way and the user drove the hilt forward, sharply jerking his hand back at the last second. The inertia of this slid the blade forward to extend from the hilt, where the lever locked the blade into place.
How Do Gravity Knives Compare to Other Types of Knives?
Gravity knives are largely legal compared to other spring-assisted types of knives, namely switchblades. Many modern folding knives have a pin near the side of the folded blade. A user can flick this pin with his thumb to open the knife one-handed. This means they can perform the same function as gravity knives. However, they still have stiff hinges. This means that someone with a gravity knife would be capable of drawing and opening his knife faster than someone with a folder knife with a pin. To that extent, they are still considered quicker and easier to use than most other legal knives.