All of which require work = energy. Differentiating momentum, energy and work (and power) can be confusing as the concepts are quite abstract, not something you can "see". While it's not a 1:1 accurate analogy, think of momentum as an object's ability to uphold its motion or resist a change in motion (related to Newton's First Law). The theoretical infinite mass, infinitely low speed car with infinite momentum would just keep going until something stopped it. Do note in all practical real world conditions at least some disruptive forces always exist so you'll never see something moving ad infinitum, even a lubricated piece of smooth steel on smooth ice will eventually stop. Also remember that to get the first car moving, energy must be consumed and work on the object done (=exert a force on it to give it acceleration). This is the maximum amount of energy or work any of the cars in the sequence are able to do. No matter how many x2 times heavier cars you have on your experiment, the last one cannot compress any bigger a spring than you could by the force you used to set the first car in motion. PS: I'm fairly certain the experiment you described hasn't been made as it doesn't really "make sense". Feel free to try it out though!