Euler's laws of motion each governed by Newton’s laws of motion, then Euler’s equations can be derived from Newton’s laws. Euler’s equations can, however, be taken as axioms describing the laws of motion for extended bodies, independently of any particle distribution. The total body force applied to a continuous body with mass , mass density , and volume , is the volume integral integrated over the volume of the body: where is the force acting on the body per unit mass (dimensions of acceleration, misleadingly called the "body force"), and is an infinitesimal mass element of the body. Body forces and contact forces acting
Gravity knife 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 ("centrifugal" force) 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