Processing 2D physics demo

// model
float ball_x = 0, ball_y = 0; // [m]
float ball_vx = 0, ball_vy = 0; // [m/s]

float FRAMERATE = 50;
float dt = 1/FRAMERATE;

void setup() {
  size(600,400);
  background(200,230,255);
}

void draw() {
  update(dt);
  // background(200,230,255);
  fill(255,150,0);
  ellipse(ball_x*50,height - ball_y*50, 20, 20);
}

void mouseClicked () {
  ball_x = 0;
  ball_y = 0;
  ball_vx = 2*mouseX/50;
  ball_vy = 2*(height - mouseY)/50;
}

void update(float time) {
  // gravity
  ball_vy = ball_vy - 9.8*time;

  // air resistance
  float speed = sqrt(ball_vx*ball_vx + ball_vy*ball_vy);
  float k = 0.1;
  ball_vx = ball_vx - k*speed*ball_vx*time;
  ball_vy = ball_vy - k*speed*ball_vy*time;

  // update position
  ball_x = ball_x + ball_vx*time;
  ball_y = ball_y + ball_vy*time;
}