In a rear-end collision the head is accelerated faster and harder than the torso, this is known as?

In a rear-end collision the head is accelerated faster and harder than the torso, this is known as Whiplash.

Explanation:

Actually, it's the tires that get stimulated quicker than a head. The head then fastens first etc creating an effect known as whiplash damage to the neck. This is both uncomfortable and can be long lasting.