Change Selected Radio based on Calculated Total

[solepixel]

I have custom calculation script and I want to change a selected radio button based on the value of the calculation. If the calculation is positive, then use selector1, otherwise, use selector2. (selector1 is checked by default)

var fTotal = this.getField("first_total").value;
var advance = this.getField("advance").value;
var charged = this.getField("charged_atcu").value;
var gTotal = fTotal - advance - charged;
if(gTotal < 0){
    this.getField("selector1").checked = false;
    this.getField("selector2").checked = true;
}
event.value = gTotal;

Can someone explain how to uncheck 1 radio and check another. I don't have any issues with making the radio buttons the same name (to keep them grouped) however, I don't even know how to control these through javascript. Any ideas?