So I’m trying to mind my p’s and q’s while learning some quantum physics, and it strikes me that nobody seems to have a unit of measure for momentum, which is of momentous importance if you want to be a dynamic character in physics.
There is a unit for it, the bole, which is a cgs unit equal to one gram (centimeter/second). But there’s scanty information about it, and everyone actually seems to be using unnamed ‘units’ like Newton-second and such.
Maybe someone, somewhere, figured that someone would try to compute the momentum of a 1 kg object traveling at a speed of 20000 km/s, and that would simply be 2 terabole to express.
My question: Is there a particularly interesting reason nobody bothers with having a specific unit of measurement for momentum?
My guess is because it’s just not necessary. It’s useful to have a unit for energy because Joules can be measured in multiple ways: in terms of mechanical quantities like mass and velocity or in terms of thermodynamic quantities like temperature and entropy.
Momentum, however, is pretty much all mechanical. Momentum will always be the result of mass and velocity, and cannot be converted into any other form.
Momentum is a vector quantity. So you need to know both it’s mass and velocity to get a meaningful measurement. Those are two variables (especially the latter), so I doubt a useful unit could be devised.
Not by much, but arguably, velocity relies on both vectors (magnitude and direction), whereas acceleration usually only needs to account for magnitude.
It is especially a variable.
I suppose I meant in context of QM, momentum will be changing far more often than the mass of particles. And, of course, mass itself is scalar.
Tell that to a student working on a problem involving a roller coaster. The direction of force usually matters, just as the direction of momentum matters.
Fact is, there’s nothing about the vector nature of momentum that precludes it having a unit of its own.
Well, I suspect the answer is the boring one: it’s not such a pain in the ass to write or say “kg m/s” (or whatever units) that everybody wants a shortcut. I guess there is some physics there, as momentum is specific to a particular object at a particular time, so it’s not generally referred to outside of specific problems. In other words, nobody says “This phenomenon is only found in ranges of momentum from 10 kg m/s to 100 kg m/s” (unlike something such as pressure, where it we do refer to particular quantities a lot). An nobody measures momentum directly (so nobody needs to label their momentumometer dial).
I honestly did not know about the bole, and thank you for drawing it to our attention.
I suspect people don’t use it because they don’t really need it, and are trying to avoid multiplying units that they don’t really need to know. g-cm/sec really says it all, and gives you the dimensions.
My counterexample is radiometry, which bristles with obscure and weirdly-named units that everyone is always forgetting the apopolication of and definition of:
