Simple test to confirm GMod's inertia unit (it's kg*m^2)

torquetest.lua

if SERVER then
    util.AddNetworkString("TorqueTest")
    util.AddNetworkString("TorqueTestEntity")

    local ply = me --If you aren't running this with luapad set this to your player entity

    local e = ents.Create("prop_physics")
    e:SetModel("models/props_junk/wood_crate001a.mdl")
    e:SetPos(ply:GetPos() + Vector( 0, 0, 100 ))
    e:Spawn()
    e:SetCollisionGroup(COLLISION_GROUP_WORLD)
    if CPPI then e:CPPISetOwner(ply) end

    local phys = e:GetPhysicsObject()

    phys:EnableDrag(false)
    phys:EnableGravity(false)
    phys:SetMass(500)
    local inertia = phys:GetInertia().z

    local lastAngVel = 0
    hook.Add("Think", "TorqueTestThink", function()
        if not IsValid(e) or not e:IsValid() then
            hook.Remove("Think", "TorqueTestThink")

            return
        end

        --39.3701 = 1 meter in units
        --Force applied is a total of 10 newtons at a distance of 1 meter from the center of the entity
        --Rotation is around the Z axis
        phys:ApplyForceOffset(phys:LocalToWorldVector(Vector(0, 1, 0)) * 5 * 39.3701 * FrameTime(), e:LocalToWorld(Vector(39.3701, 0, 0)))
        phys:ApplyForceOffset(phys:LocalToWorldVector(Vector(0, -1, 0)) * 5 * 39.3701 * FrameTime(), e:LocalToWorld(Vector(-39.3701, 0, 0)))

        local angVel = phys:GetAngleVelocity().z / 57.2958 --Convert to rad/sec
        local angAccel = (angVel - lastAngVel) / FrameTime()
        lastAngVel = angVel

        --Torque (Nm) = Inertia (kg*m^2) * AngularAcceleration (rad/s^2)
        --https://www.softschools.com/formulas/physics/torque_formula/59/
        local torque = math.abs(inertia * angAccel)

        net.Start("TorqueTest")
        net.WriteFloat(math.abs(angVel))
        net.WriteFloat(math.abs(angAccel))
        net.WriteFloat(math.abs(torque))
        net.Broadcast()
    end)

    timer.Simple(0.1, function()
        net.Start("TorqueTestEntity")
        net.WriteEntity(e)
        net.Broadcast()
    end)

    undo.Create("prop")
    undo.AddEntity(e)
    undo.SetPlayer(ply)
    undo.Finish()
else
    local e
    local angVel = 0
    local angAccel = 0
    local torque = 0

    net.Receive("TorqueTest", function()
        angVel = net.ReadFloat()
        angAccel = net.ReadFloat()
        torque = net.ReadFloat()
    end)

    net.Receive("TorqueTestEntity", function()
        e = net.ReadEntity()

        hook.Add("HUDPaint", "TorqueTestHud", function()
            if not e or not e:IsValid() then
                hook.Remove("HUDPaint", "TorqueTestHud")

                return
            end

            draw.SimpleText("AngVel: " .. math.Round(angVel, 2) .. " rad/s", "DermaLarge", 32, ScrH() / 2, Color(255, 255, 255, 255), TEXT_ALIGN_LEFT, TEXT_ALIGN_CENTER)
            draw.SimpleText("AngAccel: " .. math.Round(angAccel, 2) .. " rad/s/s", "DermaLarge", 32, ScrH() / 2 + 32, Color(255, 255, 255, 255), TEXT_ALIGN_LEFT, TEXT_ALIGN_CENTER)
            draw.SimpleText("Torque: " .. math.Round(torque, 2) .. " Nm / " .. math.Round(torque * 0.737562, 2) .. " ft-lbs", "DermaLarge", 32, ScrH() / 2 + 64, Color(255, 255, 255, 255), TEXT_ALIGN_LEFT, TEXT_ALIGN_CENTER)
        end)
    end)
end