chainsim_spring_addforce.js

    AddForce() {
        // Calculate the length of the spring.
        const dx = this.ball2.x - this.ball1.x;
        const dy = this.ball2.y - this.ball1.y;
        const len = Math.sqrt(dx*dx + dy*dy);

        // The difference between the spring's rest length and its current length
        // tells how much it is stretched or compressed.
        // Multiply by the spring constant to get the magnitude of the force.
        const displacement = len - this.restLength;
        const force = this.springConst * displacement;

        // Safety valve: if two balls are at the same location, we avoid division by zero.
        if (Math.abs(len) >= 1.0e-6) {
            // Calculate force vector = force magnitude * directional unit vector
            const fx = force * (dx / len);
            const fy = force * (dy / len);

            // Add equal and opposite forces to the two connected balls.
            this.ball1.fx += fx;
            this.ball1.fy += fy;
            this.ball2.fx -= fx;
            this.ball2.fy -= fy;
        }
    }