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Reword "undefined" to "unspecified" on the topic of units.

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To a software engineering audience, "undefined" has a strong connotation that the concept is fundamentally ill-defined or ill-behaved (c.f. "undefined behaviour").

PiperOrigin-RevId: 527192289
Change-Id: I8e81d1256c1330cd0e29c00f9913b9772d5344e6
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Saran Tunyasuvunakool
2023-04-26 01:10:17 -07:00
committed by Copybara-Service
parent d23c5e784e
commit a8d202a0c2
1 changed files with 10 additions and 10 deletions
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doc/overview.rst
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@@ -611,13 +611,13 @@ away and cause divergence.
.. _Units:
Units are undefined
~~~~~~~~~~~~~~~~~~~
Units are unspecified
~~~~~~~~~~~~~~~~~~~~~
In MuJoCo basic physical units are undefined. The user may interpret the system of units as they choose, as long as it
is consistent. To understand this, consider an example: the dynamics of a 1 Meter spaceship that weighs 1 Kg and has a 1
Newton thruster are the same as those of a 1 cm spaceship that weighs 1 gram and has a 1 dyn thruster. This is because
both `MKS <https://en.wikipedia.org/wiki/MKS_system_of_units>`__ and `CGS
MuJoCo does not specify basic physical units. The user may interpret the system of units as they choose, as long as it
is consistent. To understand this, consider an example: the dynamics of a 1 meter spaceship that weighs 1 kilogram and
has a 1 Newton thruster are the same as those of a 1 cm spaceship that weighs 1 gram and has a 1 dyn thruster. This is
because both `MKS <https://en.wikipedia.org/wiki/MKS_system_of_units>`__ and `CGS
<https://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units>`__ are consistent systems of
units. This property allows the user to scale their model as they choose, which is useful when simulating very small or
very large things, to improve the numerical properties of the simulation.
@@ -625,7 +625,7 @@ very large things, to improve the numerical properties of the simulation.
That said, users are encouraged to use MKS, as there are two places where MuJoCo uses MKS-like default values:
- The default value of :ref:`gravity<option>` is (0, 0, -9.81), which corresponds to Earth surface gravity in MKS.
Note that this does not really define system of units to be MKS, since we might be using CGS on
Note that this does not really specify the MKS system of units, since we might be using CGS on
`Enceladus <https://en.wikipedia.org/wiki/Enceladus>`__.
- The default value of :ref:`geom density<body-geom>` (used to infer body masses and inertias) is 1000, which
corresponds to the density of water in MKS.
@@ -635,9 +635,9 @@ in `Dimensional Analysis <https://en.wikipedia.org/wiki/Dimensional_analysis>`__
interpreted as MKS, then forces and torques are in Newton and Newton-Meter, respectively.
**Angles:** Although angles can be specified using degrees in MJCF (and indeed degrees are the
:ref:`default <compiler>`), internally all angles are `Radians <https://en.wikipedia.org/wiki/Radian>`__. So e.g., if we
are using MKS, angular velocities reported by :ref:`gyroscopes<sensor-gyro>` would be in rad/s while stiffness of hinge
joints would be in Nm/rad.
:ref:`default <compiler>`), all angular quantities :ref:`mjModel` and :ref:`mjData` are expressed in
`radians <https://en.wikipedia.org/wiki/Radian>`__. So, for example, if we are using MKS, angular velocities reported by
:ref:`gyroscopes<sensor-gyro>` would be in rad/s while stiffness of hinge joints would be in Nm/rad.
.. _SurprisingCollisions: