Air bag sensors do no not work by something hitting them. They work by measuring the acceleration or deceleration, in all directions, that the sensor experiences (which would be pretty much uniform for the whole car during a collision).
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They do not work in the rear end collision, because there is no sensor in the rear.
However, I guess I should have said they could trigger the side sensor, instead of hit the sensor.
They do not "work" (i.e. activate the front air bags to deploy) in a rear end collision because that would cause an acceleration. The front bags deploy in a deceleration event. What is important in the triggering of the sensor is it's acceleration or deceleration, not it's location.