class Particle {
    constructor(x, y) {
        this.position = createVector(x, y);
        this.velocity = createVector(0, 0);
        this.gravity = createVector(0, 10); // G = 9.81
        this.acceleration = createVector(0, 0);
        this.duration = 0;
        this.life = 1;
        this.mass = 1;
    }

    get heading() {
        return this.velocity.heading();
    }

    get finished() {
        return this.duration >= this.life;
    }

    applyForce(force) {
        this.acceleration.add(force.div(this.mass));
    }

    update() {
        // 重力加速度
        this.acceleration.add(this.gravity); // G = 9.81
        // 计算时间差
        const duration = deltaTime / 1000; // 秒
        this.duration += duration;
        // 计算速度差及位移
        const deltaVelocity = this.acceleration.mult(duration);
        const averageVelocity = deltaVelocity.copy().div(2).add(this.velocity);
        const displacement = averageVelocity.mult(duration);
        // 更新速度及位置
        this.velocity.add(deltaVelocity);
        this.position.add(displacement);
    }
}