Does the phenomenon of inertia exist by itself as a curious aspect of nature? No! We can see the reason for inertia in terms of energy. We have seen that motion is a form of energy; a moving mass is capable of causing things to go, work, or change. We have also learned that energy is neither created nor destroyed; it is only changed from one form to another.
Therefore, a mass at rest remaining at rest is simply to say that it will not move unless/until it gains energy. It gains energy (movement energy) from a force being applied. Conversely, a moving body remaining in motion is saying that to slow down and stop, its movement energy must be removed. On earth this generally occurs through friction, running into something, or the force of gravity.
Note and emphasize how the phenomenon of inertia reinforces the concept that energy is neither created nor destroyed.
We did the gravity lesson with my 5-year old, and a couple of weeks after, while playing with a toy hydraulic lift, he mentioned “the energy of the hydraulic pressure makes the this lift go up, because the gravity is weaker than the pressure”. He came up with that on his own, based on the lessons we have given him so far, but I hadn’t been able to expand upon it until I saw this post. Thank you for the BFSU book, thanks to the lessons my boy often wakes up excited about science, wants to learn more, and says he wants to be a scientist when he grows up. This site is a treasure trove of information, will continue to refer to it for our lessons.