Make a Bouncing Ball 

Copy this code as a sample. It also has instructions embedded in the code

int add_to_x = 3;            // set integer variable for 'x' velocity

float add_to_y = 3;          // set float variable for 'y' velocity

int ball_x = 20;             // set integer variable for initial ball 'x' placement

float ball_y = 320;          // set float variable for initial ball 'y' placement

int ball = 50;               // set integer variable for ball size

void setup() {

  size(940, 320);

}

void draw() {

  // ball = mouseY/2; // variable ball size

  background(0); 

  fill(0, 0, 255);                      // set ball color

  ellipse(ball_x, ball_y, ball, ball);  // this draws the ball

  if (ball_y>height-ball/2) {           // check if the ball is at the bottom

    add_to_y = -3;                      // change 'add_to_y' so the ball goes up

  }

  float p = add_to_y;

  if (ball_y<ball/2) {                  // check if the ball is at the top

    add_to_y = -p;                      // change 'add_to_y' so the ball goes down

  }

  if (ball_x>width-ball/2) {            // check if the ball is at the right edge

    add_to_x = -3;                      // change 'add_to_x' so the ball goes to the left

  }

  if (ball_x<ball/2) {                  // check if the ball is at the left edge

    add_to_x = +3;                      // change 'add_to_x' so the ball goes to the right

  }

  // add_to_y = add_to_y +.03;  // this line adds a little bit of gravity effect

  ball_x = ball_x + add_to_x;           // reset the 'x' value for ball position 

  ball_y = ball_y + add_to_y;           // reset the 'y' value for ball position

}