Which law of thermodynamics [first or second] applies to these two scenarios; a car engine and the human body?
The law of thermodynamics- First law states that energy can neither be created nor destroyed but can be converted from one form to another while second law states that entropy of closed system keep on increasing with time and can never be negative.
Now lets look at the car engine, its a closed system, and there is transfer of energy takes place from Heat to mechanical energy, but that energy can never be created or destroyed, so it follows 1st law of thermodynamics.
Now if we consider Human body, it is not a closed system, human body is an open system and is not subject to law of entropy or second law. But again same as car engine, human body too follow the 1st law of thermodynamics, here also energy gets converted from food energy(chemical energy) to heat energy.
Thus, its the 1st law of thermodynamics which applies to 2 systems-car engine and human body.