Define explicit units for all relevant properties

[aaronfranke]

The current draft spec does not explicitly define what units the numbers are in. Different engines use different conventions, so I think it's important to explicitly define what units the model is in, so that they can be converted as necessary.

I propose the following properties have their units explicitly defined with SI units wherever possible:

Rigid Bodies

	Type	Proposed Unit
mass	number	Kilograms (kg)
linearVelocity	number[3]	Meters per second (m/s)
angularVelocity	number[3]	Radians per second (rad/s)

Colliders

The details of the properties of the objects representing each collider shape have not been specified yet, but I propose that all of the distance units be explicitly specified to be in meters (m).

Joints

	Type	Proposed Unit
min	number	Meters (m) or Radians (rad)
max	number	Meters (m) or Radians (rad)
springConstant	number	Kilogram per second squared (kg/s²) aka Newton per meter (N/m)

For the spring constant, see https://en.wikipedia.org/wiki/Hooke%27s_law#Units_of_measurement

Scene Properties

	Type	Proposed Unit
gravity	number[3]	Meters per second squared (m/s²)

[eoineoineoin]

Yes, 100% agree; note, the glTF spec already specifies:

The units for all linear distances are meters.

All angles are in radians.

Positive rotation is counterclockwise.

https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#coordinate-system-and-units

Certainly would be good to reinforce and explicitly specify the new units.