Edit in JSFiddle

function Sphere(radius) {
    this.radius = radius;
}

Sphere.prototype.pi = 3.14;

Sphere.prototype.volume = function() {
    return Math.pow(this.pi, 3) * 3/4 * this.radius;
};

var football = new Sphere(10);

document.body.innerHTML += "<p>Volume: " + football.volume() + "</p>";

Sphere.prototype.pi = Math.PI;

document.body.innerHTML += "<p>Accurate volume: " + football.volume() + "</p>";