Systems are an essential aspect of energy analysis. Energy is always conserved but depending on the physical scenario and the system which is chosen for analysis the energy in that system may increase, decrease, or remain constant. That’s why it can be very important to precisely specify which system you are choosing.
 Who gets to choose the system? | In physics you get to be ‘master and commander’ of your choice of system. You choose the system and everything else is part of the environment. |  |
| Doesn’t everything that effects the energy story need to be included in the system? | It is certainly important to consider all of the factors that effect the energy story but in physics you get the scientific license to choose your system for energy analysis strategically. | Is there a difference between a physics ‘system’ and a biology ‘system’? |
| What are some objects which might be left out in a strategic system choice? | Sometimes an object such as a person can play an integral role in an energy story but you are not interested in investigating the full energy story of the object. If you only care about how an object transfers energy to or from another system you may want to keep that object out of your system. |  Check out this video for an example of strategically leaving out an object for the purposes of energy analysis |
| What are some objects which should be included in a strategic system choice? | In some situations it can be difficult or impossible to determine precisely how much energy accumulates in each individual object. When it is easier to determine the total energy changes of a set of objects then it may be strategically useful to include all of those objects in the your system. | Check out this video for an example of strategically including an object for the purposes of energy analysis |
References:
Examining physics teacher understanding of systems and the role it plays in supporting student energy reasoning, L. Seeley, S. Vokos and E. Etkina, American Journal of Physics 87, 510 (2019).
Energy and the Confused Student II: Systems, J. Jewett, The Physics Teacher 46, 81 (2008).