Answer:
Energy conservation.
Explanation:
The 1st Law of Thermodynamics is a statement about energy conservation. It states that 
, which means that if we <u>substract the work W done</u> by the system to the <u>heat Q given</u> to the system we get the <u>change in the internal energy</u> 
, so any excess in energy given to the system appears as internal energy, stating that energy is conserved.
Retrograde. Planets seem to move forward and then backward sometimes. This is really because we pass them as we move in our orbit but astronomers wanted to try to describe the solar system with earth at the center so elaborate models were employed.
