Centre of Mass

Newton's laws of motion, as originally stated, strictly apply to POINT masses only. Real objects, however, always have a finite, measurable size and an internal distribution of mass. The concept of the CENTRE OF MASS (c.m.) resolves this: it lets us treat any extended object as though its entire mass were concentrated at one particular point, so that Newton's point-mass laws can be applied validly to that single point, standing in for the whole (possibly complicated) extended body. This idea underlies essentially every later application of Newton's laws to real, finit …