Gravity knife

A gravity knife is a knife with a blade contained in its handle, and which opens its blade by the force of inertia or gravity. As the gravity knife requires gravity or spinning motion to propel the blade out of the handle, it differs fundamentally from the switchblade, which opens its spring-propelled blade automatically upon the push of a button, switch, or fulcrum lever. The main purpose of this opening method is that it allows opening and closing to be done one handed, in situations where the other hand is occupied. A major historical use has been in issue to parachutists to cut off their parachutes when tangled in a tree or in similar situations.

The gravity knife uses a button, trigger, or fulcrum lever to release the blade from both the open and the closed positions, and may use a side-folding or telescoping (out-the-front, or OTF) blade. While most military gravity knives utilize a locking blade design, other types may not mechanically lock open but rely instead upon friction to wedge the rear section of the blade against the interior of the handle. Factory-made gravity knives have various types of buttons, triggers, and fulcrum levers, which usually are used to release the blade from both the open and the closed positions.

One of the most recognizable gravity knives is the World War II Luftwaffe Fallschirmjäger-Messer or German air force paratrooper knife, which utilizes a four-inch (100 mm) telescoping (OTF), gravity-propelled locking blade. First produced in 1937, the FJM was issued to German flight crews and paratroops, primarily for the purpose of cutting a trapped parachutist from his rigging in case he landed with a tangled parachute, or became entangled in trees with the shroud lines. Though not intended for use as a fighting knife, the FJM's blade could be and was used as a close combat weapon when necessary.

The Luftwaffe Fallschirmjäger-Messer uses a sliding blade inside a metal gripframe, which was originally fitted with smooth wood scales, usually of beech or walnut. The blade itself is a relatively blunt spear-point, and the profile is flat ground, tapering to a utility edge. To open the blade using gravity, the user points the FJM downwards while flipping up the fulcrum-style operating lever, allowing gravity to draw out the blade to its fullest extent. Releasing the lever locks the blade into position. The FJM may also be opened by flipping the blade release lever while flicking the wrist holding the knife, causing the blade to extend. The FJM was also equipped with a folding marlinspike or awl. Primarily intended for untangling rope knots, it can also be used as a prying tool. The FJM's spike does not lock when opened and was never intended to be used as a combat weapon, though individual German paratroopers may have employed it as such.

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