Is this example of Leibniz notation sloppy?

Question

In helping a family member who is studying calculus, I was asked about the meaning of the following, which is straight out of a calculus text book (Varberg and Purcell)

$v'(t) = \frac{{\rm d}\,v}{{\rm d}t}$

I've also seen the same thing elsewhere.

Now I haven't done much calculus since my undergrad, but I've done a lot of logic and C.S.; so much, in fact, that I am now unable to read a formula without fretting about the type and binding of each variable. To my (perhaps hypercritical) eye this looks plain wrong: on the, left $v$ is being treated as a function. On the right, $v$ is being treated as a (dependent) real variable. To me, it seems the equation should be written as

$v'(t) = \frac{{\rm d}\,v(t)}{{\rm d}t}$

or as

$\dot{v} = \frac{{\rm d}\,v}{{\rm d}t}$ ,

depending on whether you want to treat $v$ as a function or as a variable.

My question: Is the text book being sloppy about notation, or is there some defensible interpretation of $v'(t) = \frac{{\rm d}\,v}{{\rm d}t}$ that I'm missing?

Answer 1

Tl;dr: They are all equal and this is a matter of preference. The notations mean the same concept, each with its respective advantages and disadvantages.

Newton's notation $\dot{v}$ is commonly used in physics and will always mean you are considering a function of time, denoted by the parameter $t$. This notation has the advantage that you avoid confusion regarding with respect to what is the function being differentiated. The disadvantage is that this is one of the cases of frozen notation: don't dare writing $\dot{v}$ for differentiation with respect to anything else but $t$.

Leibniz's notation $\frac{dv}{dt}$ is very flexible. The advantages of it are that it already tells you what is the function differentiated and with respect to what. It also adapts easily to all cases, as this is not frozen notation. The disadvantage is that there is the slight abuse you have noticed. Correctly we should write $\frac{dv(t)}{dt}$, but $\frac{dv}{dt}$ conveys the idea pretty well.

Lagrange's notation $v'$ is the most confusing of all, in my opinion. Is not entirely clear what you are differentiating with respect to. Sure, if you have a one variable function it works, but I've seen people write $v'$ when $v$ is a function of space and it meant differentiation with respect to time, without mentioning it. If abused it can lead to great trouble. The chain rule in this notation states that $(f(g(x))' = f'(g(x)) g'(x)$, but what if you omit the parameters? It becomes $(f \circ g)' = f' g'$, which is horrible (it may even confuse students with the product rule).

Answer 2

I disagree with your descriptions of the meaning of the various uses of $v$; in fact, I would flip them! I think that what I say below describes the common mathematical perspective, but it could be that I am wrong; for example, Mark Fantini above (https://matheducators.stackexchange.com/a/4282/2070) seems to disagree.

In mathematical convention (which differs from some computer-science conventions), a 'bare' name $v$ refers to a function, whereas an 'argumented' name $v(t)$ refers to its value. It is very, very common to abuse this, and so to refer to "the function $\sin(x)$" rather than "the function sin" (which looks terrible, or perhaps theological) or "the function $x \mapsto \sin(x)$", especially in contexts where the function has no name (like "the function $x^2$").

Thus, I would argue that combining convention with precision (which is not necessarily desireable!) would dictate writing $v' = \frac{\text dv}{\text dt}$ (or $\dot v$ if you prefer it in place of $v'$), or, if you really want, the cumbersome and fairly ugly notation $v'(t) = \frac{\text dv}{\text dt}(t)$. (Another common approach to indicating a value to be plugged into a derivative is to write something like $\left.\frac{\text dv}{\text dt}\right|_{t = 7}$ for $v'(7)$.)

I find an approach that rarely mentions explicit arguments particularly valuable in a differential-equations course, where, for example, we might have a height $y$ depending on a position $x$, which in turn depends on a time $t$—in which context it is much easier to get to the bottom of the physical meaning if one writes just $\frac{\text dy}{\text dx}$ or $\frac{\text dy}{\text dt}$ rather than, say, $\frac{\text dy(x)}{\text dx}$ or $\frac{\text dy(x(t))}{\text dt}$.