// Learning Processing
// Daniel Shiffman
// http://www.learningprocessing.com

// Example 5-9: Simple Gravity

float x = 100;   // x location of square
float y = 0;     // y location of square

float speed = 0;   // speed of square

// A new variable, for gravity (i.e. acceleration).   
// We use a relatively small number (0.1) because this accelerations accumulates over time, increasing the speed.   
// Try changing this number to 2.0 and see what happens.
float gravity = 0.1;  

void setup() {
  size(200,200);

}

void draw() {
  background(255);

  // Display the square
  fill(175);
  stroke(0);
  rectMode(CENTER);
  rect(x,y,10,10);
  
  // Add speed to location.
  y = y + speed;
  
  // Add gravity to speed.
  speed = speed + gravity;
  
  // If square reaches the bottom
  // Reverse speed
  if (y > height) {
    // Multiplying by -0.95 instead of -1 slows the square down each time it bounces (by decreasing speed).  
    // This is known as a "dampening" effect and is a more realistic simulation of the real world (without it, a ball would bounce forever).
    speed = speed * -0.95;  
  }
}