To learn the definition of the center of mass for systems of particles and be able to locate it. imagine throwing a rock upward and away from you. with negligible air resistance, the rock will follow a parabolic path before hitting the ground. now imagine throwing a stick (or any other extended object). the stick will tend to rotate as it travels through the air, and the motion of each point of the stick (taken individually) will be fairly complex. however, there will be one point that will follow a simple parabolic path: the point about which the stick rotates. no matter how the stick is thrown, this special point will always be located at the same position within the stick. the motion of the entire stick can then be described as a combination of the translation of that single point (as if the entire mass of the stick were concentrated there) and the rotation of the stick about that point. such a point, it turns out, exists for every rigid object or system of massive particles. it is called the center of mass. two particles of masses m1 and m2 (m1