//function to remove cannon balls if they are out of screen or remain stationary for a long time
void removeUselessBall(CannonBall ball, ArrayList ballList)
{
if (ball.getPosition().x < leftBoundary || ball.getPosition().x > rightBoundary || ball.getPosition().y < bottonBoundary)
{
//when out of scree, remove the ball from the ball list and destroy it
ballList.Remove(ball);
Destroy(ball.GetGameObject());
}
else if (Mathf.Abs(ball.getVelocity().x) < 0.0001f)
{
//the ball's x-velocity is really small (caused by friction), remove the ball from the ball list and destroy it
//note that it must been a long time for the ball staying on the ground to have such a small x-velocty
ballList.Remove(ball);
Destroy(ball.GetGameObject());
}
}