Originally Posted by
hollywood It's the chain rule for multiple variables - if f is a function of two variables x and y, each of which is a function of t, then:
$\frac{df}{dt} = \frac{\partial f}{\partial x}\frac{dx}{dt}+\frac{\partial f}{\partial y}\frac{dy}{dt}$
Changes in t cause changes in both x and y, which each cause changes in f, and the effects are additive.
To understand what follows later, it might help to break r into x, y, and z. Then you'd have 4 terms, but the x, y, and z terms should combine to that $v \cdot \nabla C$.
- Hollywood