In federal court, when is work product discoverable?

Hypo 1: Bob owns an auto mechanic shop. Sam has his car serviced at Bob's shop and, days later, is involved in a horrible crash. Sam hires an attorney to sue Bob. Bob's attorney prepares for the lawsuit by diligently reviewing all the records Bob has in his shop related to Sam. He compiles these documents into a file and, as he reads through them, he makes notes in the margin like, "This looks bad," and "Be sure to check whether the mechanic who worked on Sam's car was covered by the insurance." A couple days later, Sam's attorney subpoenas all records related to Sam's car being worked on at Bob's shop. Before the records can be sent to Sam's attorney, Bob's shop is hit by a meteor, immediately vaporizing Bob's shop, a handful of his employees, and everything in a 500-meter radius. Result: It is vital to Sam's case that he have copies to the records about how Bob's shop worked on his car and, without them, it is unlikely Sam will be able to bring a viable suit against Bob. It wouldn't be very fair to deny Sam an opportunity at justice, and so the court will likely allow Sam's attorney to request the file that Bob's attorney had created based on the records that no longer exist in any other form. However, while the court will allow Sam's attorney access to the substantive parts of the records in order to build their case, the court will not allow them to sneak a peek at the strategies, opinions, conclusions, and other impressions made by Bob's attorney (such as the notes that he made in the margins of the records, or any memos that may have been drafted based on what was learned from them, etc.).