Supersonic Man

May 30, 2016

the dimensionality of torque

Filed under: Hobbyism and Nerdry,thoughtful handwaving — Supersonic Man @ 10:33 am

Just clearing up a little thing that’s always bugged me…

“Torque” is rotational force: the measure of how hard you’re twisting something.  We don’t measure it directly; we can only take a measurement of it by gauging the amount of linear force that it exerts at a given distance from the axis of rotation.  Because of how levers work, this force is high if you’re close to the axis and low if you’re a long ways off.  The linear force times the length of the lever-arm from the center equals the torque.  So we measure it in foot-pounds or newton-meters. If you double the length, you halve the force, and the product is the same, so the particular force and length numbers don’t matter — only the combined value does.

But wait — force times length equals work.  Is torque in some sense the same thing as energy?  Hell no.  Torque is static; you aren’t doing work until you make it move.  If you exert force at the end of the lever through some distance, thereby rotating something while exerting that torque, that’s work.  If you continue to do so over more distance, that’s more work.  Now doubling the lever length halves the force at the end, but doubles the distance it has to travel for a given angle of turn, meaning that the particular lengths don’t matter — the work is the same for a given angle of turn.

Work equals torque times amount of rotation.  The correct SI unit of torque is not newton-meters, but newton-meters per radian.

So why don’t people mention the radians?  Because radians are defined as arc-length over radius: a distance divided by a distance.  This cancels out to a pure number, a dimensionless ratio.  Or so it is traditionally argued.

But we’ve clearly lost something there.  The rotation is a very real physical thing, and its presence or absence makes all the difference, as we’ve seen, when relating torque to work.  For that matter, just trying to solve for how far a projectile will fly, given its speed and angle, falls apart dimensionally if you say the angle is just a number.  I think it’s time to acknowledge that when we divide a curved distance by a straight distance, what’s left is the curvature, and this should be treated as an independent dimension in its own right.

Traditional analysis regards mass, distance, and time as the only fundamental dimensions; since then we’ve added quantities such as electric charge, and (when in a generous mood) various arcane quantum properties for which the universe seems to have conservation laws, such as quark type and “color”, and whatever it is that is shared by electrons and electron-neutrinos, but distinguishes them from muons and muon-neutrinos.  But these “material” values, as we might call them, don’t cover everything we need.

Some dimensional analysis enthusiasts have been saying that angles may not be the only neglected metric.  A minority even argue that inertial mass, gravitational mass, and “substantial” mass (whatever that is) are at least two and maybe three subtly distinct dimensions which just happen to always coincide in value.  (And indeed, the close relationship between energy, gravity, and inertia is still a mystery, despite apparent confirmation of the Higgs boson.)  But even without that, we all too easily forget that length is not a single dimension when dealing with vectors in space — each axis is separate, just as you’d think from hearing the word “dimension” in the first place.

Donald Siano has shown that if you get sophisticated enough with vector dimensions, it can express the concept of a measure of angle or rotation as a derived quantity.  He even shows that mathematical functions that we think of as pure numbers are incommensurable: for instance, sine and cosine are dimensionally distinct, and it makes no sense to add a cosine value to a sine, or (in further extensions by others) a logarithm to anything not logarithmic.  Such mathematical expressions cannot correspond to anything physical, and indeed, you should not normally ever see them when doing even the most abstract math.  So if we go for the full potato on using his “orientational” analysis, we don’t need a separate dimension just for angles.  But that’s quite cumbersome for everyday use — if we don’t want to be constantly doing 4×4 matrix arithmetic to keep our dimensions straight, we can just treat angularity as a dimension.

On the opposite side from those who want to nitpick subtly distinct properties of mass, some argue that the universe’s fundamental constants mean that quantities such as time and distance and mass and charge really are all commensurable, and at bottom there’s only one fundamental dimension (probably best expressed in our terms as energy), and all natural laws are dimensionless.  If so, so what, I say — that approach makes dimensional analysis less useful rather than more so. And if you think about vectors, space and time are still incommensurable, especially since in relativistic geometry, lengths on a timelike axis are imaginary numbers.

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1 Comment »

  1. Here’s another dimensional abuse I want to complain about: the way they use the term “specific impulse” in rocketry. What it actually means is the amount of velocity change that a given mass of fuel can give to an equal mass of material being pushed. In short, it’s simply the exhaust velocity of the flame coming out of the bottom of the engine. But rocketeers don’t like to express it as a velocity; they express it as a span of time. “SpaceX’s Merlin 1D engine, burning RP-1 kerosene and LOX, has an ISP of 281 seconds in sea level air, or 311 seconds in vacuum.” They do this by conflating mass with weight, meaning you have to multiply the time by Earth’s gravity to get the true impulse.

    One might suppose that their excuse is that they’re always thinking about lifting stuff up against gravity. But it’s been claimed that the real reason has something to do with finding common ground with how airplane engine makers talk about specific impulse. (In those engines it doesn’t equate to exhaust velocity — it’s more complicated.) That does’t make a ton of sense to me. Either way, let’s use velocity, dammit, because that’s what it actually is.

    Comment by Supersonic Man — April 23, 2017 @ 4:01 pm | Reply


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