function calculate() {
	var m1 = document.getElementById("m<sub>1</sub>").value;
    var m2 = document.getElementById("m<sub>2</sub>").value;
    var r = document.getElementById("r").value;
	var f;
	if(m1 != "" && m2 != "" && r != ""){
		r = parseFloat(r);
        m1 = parseFloat(m1);
        m2 = parseFloat(m2);
		f = (6.67300 * Math.pow(10,-11) * m1 * m2)/(r*r)
		document.getElementById("Force").innerHTML = "Force: " + f + "(N)";
	}
}