Friction disappears when manually applying gravity

[josephrocca]

I have a feeling I'm missing some fundamental understanding here, but when I edit the force on an object manually (e.g. by setting body.force.y += ... to manually apply gravity), the body suddenly loses all friction.

Is this normal behavior? I should note that I'm not using preStep, but am instead simply adding the forces before I call world.step(). I could rearrange my code structure to use preStep but it would be quite inconvenient.

For reference, here's how I'm manually applying gravity:

   var g = this.settings.gravity;
   for(var i = 0; i < this.world.bodies.length; i++) {
      var b = this.world.bodies[i];
      if(b.type === CANNON.Body.DYNAMIC) {
         b.force.y += b.mass*g;
      }
   }
   //then...
   this.world.step(this.settings.timeStep);

Thanks! Any hints at all would be appreciated.

[schteppe]

Are the bodies in contact? Maybe they lose the contact when you apply the force?

[josephrocca]

Thanks for the quick reply man :) Yep, they're in contact. It's a bunch of cubes that fall onto a heightfield. When I apply gravity using world.gravity they fall, bounce and grind to a halt. When I manually apply gravity, they fall, bounce and slide around for a fair while before eventually settling in the low points of the heightfield.

If there's nothing that immediately comes to mind I'll do some more testing and report back.

[schteppe]

Oh, I know now.

When the friction is applied, the max friction force (mu * normalForce) is approximated with mu * mass * gravityVector.length(). When gravity is zero, then the max friction force is also set to zero.

I solved this problem in p2.js by adding a .frictionGravity property to the world, that you can set to a custom value that will override gravityVector.length() when calculating the friction. I guess this should be ported to cannon.js as well.

You can get around this problem by setting the gravity vector to something non-zero and then subtract that value from each body.force before each step(). Then apply your custom gravity.

   var g = this.settings.gravity;
   this.world.gravity.set(0,g,0);
   for(var i = 0; i < this.world.bodies.length; i++) {
      var b = this.world.bodies[i];
      if(b.type === CANNON.Body.DYNAMIC) {
         b.force.y -= b.mass*g; // this will make the net gravity zero

         // apply your custom gravity here

      }
   }
   //then...
   this.world.step(this.settings.timeStep);

Sorry about this mess, should have thought of this earlier.

[josephrocca]

Great! Thanks a heap - it's all working well now :)

[schteppe]

No problemo buddy. Let's keep the issue open to remind me about porting the p2.js .frictionGravity stuff :)