Short answers, pulled from the story.
Stock variables represent quantities existing at a specific instant, such as equipment or buildings on the 31st of December 2004. Flow variables measure activity over an interval of time, like gross domestic product which functions as dollars per year.
Irving Fisher formalized the concept of capital as a stock variable in the early twentieth century. This work established the foundation for distinguishing accumulated entities from movement rates in economic theory.
Accountants calculate turnover ratios by dividing total flow by average stock during a given period. These ratios reveal how many times inventory rotates within that timeframe.
Polish economist Michał Kalecki called economics the science of confusing stocks with flows around 1936. Joan Robinson quoted his caustic observation decades later in a Cambridge Journal article from 1982.
The change in physical capital from the 1st of January 2010 to the 1st of January 2011 equals three machines per year. This difference represents the net investment flow during that specific twelve-month span.