float g = 0.8 * 2.0;
float l = 10.0;

float t = 0;

void
setup()
{
  size(320, 240);
  noStroke();
  smooth();
}

void
loop()
{
  float omega0 = sqrt(g / l);
  float theta0 = 2.0;
  
  float theta = theta0 * cos(omega0 * t); 

  float x = (width / 2) + l * sin(theta) * 10.0;
  float y = (height / 2) + l * cos(theta) * 10.0;

  fill(0xff, 0xff, 0xff, 0x33);  
  rect(0, 0, width, height);

  fill(0xff, 0, 0);
  ellipse(x - 5, y - 5, 10, 10);
    
  t += 0.1;
}