Total Derivative

#Math
Video Link
The derivative of a variable with respect to the change of all other variables. Only works if our function is reducible to just a function of the variable of interest

In the top example, we can find the total derivative $\displaystyle \frac{\mathrm{d}u }{ \mathrm{d}t}$since it's all dependent on $\displaystyle t$ but in the bottom example, there is no total derivative since it depends on $\displaystyle t$ and $\displaystyle z$, which are independent from each other