The two derivatives are related to each other: where, property (temperature, concentration, velocity, etc) If  : Euler’s acceleration formula. may be recognized as acceleration or particle-acceleration. It represents a physical acceleration. For example, one can write   (Newton’s 2nd law of motion) All it means is that the Newton’s 2nd law of motion, which is generally applied for a solid object, can also be applied or written for a fluid element or fluid flow! It also follows that  Physical mass cannot be created or destroyed !! We will see later, can be, however, zero or non-zero.