Why does the head have it's own inertia during whiplash

[hongiddong]

I was wondering why the head has it's own inertia during whiplash when the person is rear ended by another car?

I thought that the head would be moving forward also because it is part of the system of the chair being impacted to move forward?

 

[oz93666]

imagine a stationary car being hit from behind ... the seat will be pushed forward , along with the body sitting in it ... the shoulders move forward , pushed by the seat , the head , if not in contact with the head rest, will tend to remain stationary ,the only way forward momentum can be imparted to the head is through the neck ...so the shoulders move forward , head lags behind ... strain on the neck.
As you say if the head is in contact with the headrest no whiplash.

 

[OldYat47]

Every little bit of you (and the vehicle) has its own bit of inertia. Even a rigid body reacts differently to impact depending on where the center of mass of the rigid body is and where the impact force (or forces) is applied. oz93666 presented a good thought experiment.