p2.js how to implement force?

[Jevito]

hello a need help with p2 if anybody of you, provide me some information about using force I'd appreciate, I'm following the example

I'm trying to use this code postStep but what I don't understand where to put that code if is inside animation loop or outside the game loop, I tried different ways but it didn't work for me. I'm using phaser's game loop (update) for my main loop.

world.on('postStep', function(event){
    // Add horizontal spring force
    circleBody.force[0] -= 100 * circleBody.position[0];
});