function fallDown(apple) {
    if (!(IsPlaying && apple instanceof HTMLElement)) {
        return;
    }
    //store the current top position for future refrence.
    apple.setAttribute('data-top', apple.style.top);
    //change the top position, this is animated using transition property in CSS
    apple.style.top = "380px";
    //increase score
    score = score + 5;
    //show the score by calling this function
    renderScore();
    //hide the apple after it reaches the ground by calling this function
    hideFallenApple(apple);
}
Makes the provided element fall down by changing the top property.
@param {HTMLElement} apple