How to create a simple gravity engine on javascript

Question

I'm trying to make a simple animation in javascript using P5.js library. I would like to make a ball appear at some height and then let it fall down and make it bounce until it comes to a stop.

I'm not looking for a external library, just P5.

My code is this:

function Ball() {
    this.diameter = 50;
    this.v_speed = 5;
    this.ypos = height/2 - 100;
    this.xpos = width/2;

    this.update = function(){
        this.ypos = this.ypos + this.v_speed;
        this.ypos = constrain(this.ypos, this.diameter/2, height-this.diameter/2);
    }

    this.show = function(){
        ellipse(this.xpos, this.ypos, this.diameter);
        fill(255);
    }
}

var ball;

function setup() {
    createCanvas(600, 600);
    ball = new Ball();
}

function draw() {
    background(0);
    ball.update();
    ball.show();
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/0.6.1/p5.js"></script>

Can someone help me? Thanks a lot

Answer 1

First you have to set the starting speed to 0 and define a gravity:

this.v_speed = 0;
this.gravity = 0.2;

A working update method, which can be directly applied to your example, looks like this:

this.starty = height/2 - 100;
this.endy = height-this.diameter/2;

this.update = function(){

    this.v_speed  = this.v_speed + this.gravity; 
    this.ypos = this.ypos + this.v_speed;
    
    if (this.ypos >= this.endy){
    this.ypos = this.endy;
        this.v_speed *= -1.0; // change direction
        this.v_speed = this.v_speed*0.9; 
        if ( Math.abs(this.v_speed) < 0.5 ) {
            this.ypos = this.starty;
        }
    }
}

The key is to reduce the speed and to change the direction when the ball bounces on the ground:

this.v_speed *= -1.0;
this.v_speed = this.v_speed*0.9;

See also Bouncing Balls, struggling with getting more than 1 (processing).

See the example, where I applied the suggestions to your original code:

function Ball() {
    
  this.diameter = 50;
      this.v_speed = 0;
      this.gravity = 0.2;
      this.starty = height/2 - 100;
      this.endy = height-this.diameter/2;
      this.ypos = this.starty;
      this.xpos = width/2;

      this.update = function(){

          this.v_speed  = this.v_speed + this.gravity; 
          this.ypos = this.ypos + this.v_speed;
          
          if (this.ypos >= this.endy){
            this.ypos = this.endy;
              this.v_speed *= -1.0; // change direction
              this.v_speed = this.v_speed*0.9; 
              if ( Math.abs(this.v_speed) < 0.5 ) {
                  this.ypos = this.starty;
              }
          }
      }

      this.show = function(){
          ellipse(this.xpos, this.ypos, this.diameter);
          fill(255);
      }
}

var ball;

function setup() {
    createCanvas(600, 600);
    ball = new Ball();
}

function draw() {
    background(0);
    ball.update();
    ball.show();
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/0.7.3/p5.js"></script>

Demo