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What are the units of force and mass and velocity?

Posted: Mon Dec 26, 2022 6:41 pm
by Brian Beuken
Small question that is probably obvious but I can't see a clear answer in the forums.

Im trying to work out what the basic unit of mass is, I have read 1kg = 1 unit..

Which would be fine... but then what is the unit of force, newtons or pounds? And velocity, metres per sec, km?

I can't quite seem to get the numbers to work out when I am trying to apply thrust to a 100,000 mass unit ship, with only a few 10's of unit thrusts, that isn't quite fright?

Of course all this are relative to scale and view, but It would be helpful if I could get a sense of what the engine takes 1.0f to be for these values to allow me to make my motion of different size and mass ships seem more realistic as they accelerate due to trust from engines.

any insight is much apprecaited.

Re: What are the units of force and mass and velocity?

Posted: Fri Jan 06, 2023 4:45 pm
by drleviathan
I believe the units of force are Newtons: kg * m / sec^2 because the simulation step expects the duration to be in seconds. You can always use different units by scaling the substep duration and then tweaking your own mass and distance units accordingly. This can sometimes be handy when your world and objects are very big and your speeds are very high (e.g. for spaceships). The problem with using very large objects without scaling your units is the collision margin of shapes are effectively tuned (to avoid tunnelling and interpenetration) for human sized objects moving at human-experience speeds (e.g. most games).

BTW, I advise game developers to NOT apply force/torque and instead blend-in linear/angular velocities. The logic is simpler and you avoid some pitfalls. I see in the description of your video that you tried "calculating linear velocities" yourself but they were not as smooth as you would like. Perhaps you weren't doing it right? In case it helps: here are some threads where I discuss the technique:


Re: What are the units of force and mass and velocity?

Posted: Thu Mar 16, 2023 2:27 pm
by Brian Beuken
thank you this is helpful.