- #36
etotheipi
Roses are red,
violets are blue,
I think 50V is being a little condescending,
how about you?
violets are blue,
I think 50V is being a little condescending,
how about you?
One of the answers given there suggests an important distinctionPeroK said:There's a similar discussion here:
https://physics.stackexchange.com/questions/37881/why-is-torque-not-measured-in-joules
That is, the torque--regarded as a generalized force--has units of Joules/radian.BMS said:https://physics.stackexchange.com/a/93735/148184
Yes, torque has units of joules in SI. But it's more accurate and less misleading to call it joules per radian.
...
[itex]\displaystyle \tau=\frac{dE}{d\theta} [/itex].
I didn't claim that torque and energy are the same. In the SI they have the same units, which is ##\text{N} \text{m}##, and of course quantities that are measured in the same units need not be the same, as your examples show.Vanadium 50 said:Torqua is not energy and torque and energy do not have the same units. They do have the same dimensions, md2t-2, but that is not the same thing. Physics is not algebra. One joule (of energy) does not equal one Nm (of torque).
If you want another example, strain (Δx/x) and angle (arc length/radius) are both dimensionless. But you can't equate them, even if 1 = 1.
Hm, but rad is just 1. Angles, measured in radians, are dimensionless. You can of course also measure them in degrees or gons.robphy said:One of the answers given there suggests an important distinction
between the units for energy and the units for torque,
as well as making contact with the OP's question.That is, the torque--regarded as a generalized force--has units of Joules/radian.
This complements the distinction to energy being a scalar and torque being thought of as a bivector.
It appears that these above (arguably more fundamental) constructions
are (knowingly or unknowingly) reduced to the simplified forms we see in introductory physics.
(Sometimes, it seems that "taking advantage of symmetries" blurs the real distinctions between these constructions.)
The OP might appreciate the work of Enzo Tonti
https://en.wikipedia.org/wiki/Tonti_diagram
in helping to clarify the structure of physics theories.
From a practical point of view, I would agree...vanhees71 said:Hm, but rad is just 1. Angles, measured in radians, are dimensionless. You can of course also measure them in degrees or gons.
I think there's an endless discussion of this, i.e., whether writing rad in the sense of a unit for angles is consistent with the rules of the SI (or any other system of units of course) or not.robphy said:From a practical point of view, I would agree...
However, from a structural point of view,
I would agree that "rad is equal to 1" within some system of units [akin to a choice of coordinates?]
or within some context [where it is declared to be so by introducing additional structure]
... but, at another more fundamental level, they are distinct... which may not be adequately captured by a system of units where "rad is equal to 1".
I think there are seeds of this "structure" (which I can't put my finger on) in this discussion
https://golem.ph.utexas.edu/category/2006/09/dimensional_analysis_and_coord.html
but I haven't had the time to go through it.
We find it convenient to use "rad" in a calculation, then treat it as "1".
But we wouldn't use "1" in another calculation, then treat it as "rad".
So, obviously, there's something else going on.
How can this be formalized?
For example, how would these "rules" be encoded in (say) a computer program?
Your equation would hold in at least one particular case: When the momentum change in time is zero and there is actually no force field acting on the body, so the potential energy is also zero. In all other cases, it would just be a blunder, I guess.baw said:Summary:: Why ##\frac{d}{dt}\frac{\partial T}{\partial \dot{q}}## instead of ##\frac{dT}{dq}##?
Why, in lagrangian mechanics, do we calculate: ##\frac{d}{dt}\frac{\partial T}{\partial \dot{q}}## to get the (generalised) momentum change in time
instead of ##\frac{d T}{dq}##?
(T - kinetic energy; q - generalised coordinate; p - generalised momentum; for simplicity I assumed that no external forces are applied)