make inputfield active on click of radio button

I have one radio button and one inputfield..I am trying to achieve something like.if i click on the radio button , the input field should get active and user should be able to enter the value..If the radio button is not selected ..the field should be availabe but it should be disabled.

I have written something like below..but its not working

<input onclick ="document.getelementbyid('datepurchased').disabled =true;" type = "radio" name = "abc"></input>   
<apex:inputField id="datepurchased" value="{!lstRMA[0].Date_Purchased__c}"/>