Basically, what I want to know is this: Does Unity use SI and SI-derived units for its measurements?
From what I've been able to find about Unity,
Length is measured in meters,
Time is measured in seconds
Is mass measured in kilograms? Is force measured in Newtons? Does changing the mass value of a rigd body affect its density (kgm^3) or mass (kg)
What about other units, like intensity?
Length isn't meters, it's whatever you say it is...1 unit can be a millimeter or a mile. Likewise, mass is arbitrary. A mass of 1 can be an ant or a planet. It's all relative--an object with a mass of 2 is twice as heavy as an object with a mass of 1, which is the only thing that counts.
Unity is technically just in units. But these units are nearly identical to SI units. So gravity would still be 9.8units/sec^2, and 1 mass unit = 1kg.
Changing mass should affect density as well. You can also use `rigidbody.SetDensity()` to assign the mass based on a given volume and mass.
I think that answered all your questions.
Although this thread is ancient, I just want to point out that size does matter in relation to the constant acceleration of gravity because the larger objects are in your scene, the slower they will appear to fall. If you had a 1 meter diameter ball that you drop from a height of 50 meters above a plane, it will take a little more than 3 seconds to fall this distance. If you had a 10 meter diameter ball that you drop 500 meters on to a plane scaled a factor of 10 you can reposition your camera to make these scenes look identical, but in the second version the ball will appear to be moving much slower and take will in fact take much longer to fall to the plane. Obviously this is because it has a longer distance to travel, but this illustrates how the scale of your scene is quite important. Although I haven’t worked in Unity much I think you can change the force of gravity, which is nice so you could then use objects of any scale and adjust gravity accordingly. I’m not sure what effects there would be with other physics settings though.
Actually, all objects are affected by the same forces of gravity regardless of density, which is why the force of gravity as measured on earth has no parameter for mass. To illustrate this point astronauts actually dropped a feather and a hammer whilst on the moon, and they did indeed hit the ground at the same time. The differences in atmosphere are derived from the surface area, friction and of course if two objects have the same surface area and friction, the denser object will fall faster as it has more kinetic energy and therefore exerts a greater force on the air, and is able to displace air at a greater rate.