Friction units?

[JoshKlint]

What is the units of measurement for friction in dCustomHinge::SetFriction and dCustomSlider::SetFriction? Is it Newtons?

Is there a unit for the dCustomJoint::SetStiffness value? This is how "strongly" the joint holds together, right?

[Julio Jerez]

friction is in unit for force for linear joints and units of torque for angular joints

a unit for force in the MKS system is a newton (kg * m / sec^2)
a unit of torque is Newton-meters.

as for stiffness, the name has different meaning for different equations, but is general it means how much a dependent variable changes when the independent variable change by a small value.

in 3.xx it is call stiffness, but in 4.xx that name was changes to regularization.
I explaned this before.

but for a liner system is you have an equation of the for
x = A * y

if A is very, very small. say A = 0.0001

say x = 1
if you solve for y you get

y = 1.0 / 0.0001 = 10000

now say x in increment by 0.1 so it is now 1.1

solving again y again you get
y = 1.0 / 0.00001 = 11000

a change of 1000 for a small value.

although that example seem very trivial, when the linear system is a set of equation, them it is no so easy for determine the degree of stiffens for the system and for that you need to have a deeper understanding of lineal algebra that I am no going to go here.
but if you want to know more you can read about ill condition system in Wikipedia.

the important part is that so mathematicians for come up with approximation to these kind of problems, which simple find an approximation by adding a relxation values to the diagonal

so in the example about, if the is you set r (the relxation) to 0.01 for example

the equation becomes

y = A * x + r * x = (A + r) * x

now the equation becomes

y = (0.0001 + 0.01) * x

so for x = 1 you get

y = 1 / 0.0101 = 99.00

for x = 1.1
y = 1.1 / 0.0101 = 108.9

an much smoother behaviors. but in both cases, the solution are smaller that the original.
so the trick is to add a penalty to the right size that make of for the error. and that's done with a implicit spring damper.

In newton 4 we no longer name r stiffness, we just caller regularizer which is the name use in the literature of linear algebra.

r is a dimensionless factor because they are just a scale factor of the main diagonal of the mass matrix. and the main diagonal has different units based of if it was linear or angular.

[JoshKlint]

Thank you:
https://www.ultraengine.com/learn/Joint_SetFriction