what is a gravity knife
A gravity knife is a knife with a blade contained in its handle, and which opens its blade by the force of gravity. As the gravity knife requires gravity or rotary motion to propel the blade out of the handle, it differs fundamentally from the switchblade knife , which opens its spring-loaded blade automatically at the press of a button, switch, or fulcrum lever. The main purpose of this opening method is that it allows opening and closing with one hand., in situations where the other hand is busy. Thus, they have historically been issued to skydivers for cutting trapped lines, such as lines tangled in trees, an important potential use of the gravity knife.