问在box2d/matter.js中找到运动的正常和切线分量。

EN

// module aliases
var Engine = Matter.Engine,
    Render = Matter.Render,
    World = Matter.World,
    Bodies = Matter.Bodies,
    Body = Matter.Body,
    Vector = Matter.Vector,
    Composite = Matter.Composite,
    Composites = Matter.Composites,
    Constraint = Matter.Constraint,
    Events = Matter.Events;

// create an engine
var engine = Engine.create();

// create a canvas
var canvas = document.getElementById("maincanvas");

var render = Render.create({
    element: document.body,
    canvas: canvas,
    engine: engine,
    options: {
        background: "#fff",
        height: 400,
        width: 400,
        wireframes: false,
    }
});

engine.world.gravity = {x:0, y:0};

// Create a soft body composite, see 
// http://brm.io/matter-js/docs/classes/Composites.html#method_softBody
var softbox = Composites.softBody(100,100,2,2,40,40,true,1);

World.add(engine.world, softbox);


// This functions makes some constraints move.

function pulse(composite) {
  var allcons = Composite.allConstraints(composite);
  allcons[0].length += 5;
  allcons[4].length += 5;

  // Set a timeout to make the constraints short again
  setTimeout(function (cons) { cons.length -= 5;}, 1000, allcons[0]);
  setTimeout(function (cons) { cons.length -= 5;}, 1000, allcons[4]);
}

setInterval(pulse, 2000, softbox);

function applyForcesOnEdge() {
    var allcons = Composite.allConstraints(engine.world);

    allcons.forEach( function(cons, index) {
        // Edge speed defined as the average of both connected body's speed. 
        var constraintspeed = Vector.div(Vector.add(cons.bodyA.velocity, cons.bodyA.velocity),2);
        if (constraintspeed.x != 0 && constraintspeed.y != 0) {
          console.log(constraintspeed); // How to get the tangential and normal components from this?
        }
    });
}

Events.on(engine, 'beforeUpdate', function () {
  applyForcesOnEdge();
})


Engine.run(engine);

Render.run(render);