chainsim_update.js

    Update(dt) {
        var b, s;

        // Calculate the force vectors acting on all the balls:
        // both from springs and from gravity.

        // Start out with just the gravitational force on each ball.
        for (b of this.ballList) {
            b.fx = 0.0;
            b.fy = b.mass * this.gravity;
        }

        // Go through all the springs and calculate
        // the forces on the balls connected to their endpoints.
        // There will be equal and opposite forces on each pair.
        for (s of this.springList) {
            s.AddForce();
        }

        // Now all the forces are correct.
        // Use the forces to update the position and speed of each ball.
        const friction = Math.pow(0.5, dt / FrictionHalfLifeSeconds);
        for (b of this.ballList) {
            if (b.anchor === 0) {       // skip anchors, because they don't move
                // F = ma, therefore a = dv/dt = F/m.
                // dv = dt * F/m
                let dvx = dt * b.fx/b.mass;
                let dvy = dt * b.fy/b.mass;

                // Update the position using the mean speed in this increment.
                b.x += dt * (b.vx + dvx/2.0);
                b.y += dt * (b.vy + dvy/2.0);

                // Update the ball's speed. Apply friction to gradually reduce energy.
                b.vx = (friction * b.vx) + dvx;
                b.vy = (friction * b.vy) + dvy;
            }
        }
    }