[ugh]

All kinds of laptops have had accelerometers since the early 2000s or so. (IBM ran commercials for their ThinkPads with “airbag” all the time. Yep, this was already built into laptops when IBM still sold ThinkPads.) They stop the hard drive when accelerations get too extreme.

[lloeki]

Yet having the accelerometer made usable by software is actually nice. It was (and still can be, sometimes in HD themselves) often a hardwired feature. Also, there's quite a gap in sensibility between detecting a drop and registering subtle variations of the acceleration vector.

[Isamu]

I thought they stopped the drive when the acceleration is zero (freefall). If the acceleration is high, it's probably too late.

[ugh]

That makes a lot of sense. Fun fact: The astronauts on the ISS use ThinkPads [0]. They would have to disable the accelerometer if their laptops had one.

(I’m now just going to claim that zero was the extreme I was referring to.)

[0] http://de.wikipedia.org/w/index.php?title=Datei:Susan_Helms_...

Edit: The laptop in question — model A31p from 2003 — doesn’t have an accelerometer. http://www.thinkwiki.org/wiki/Category:A31p

[logophobia]

Zero acceleration only happens after an object has been falling for a long time (more then a few seconds) and the friction is in balance with gravity. Zero acceleration also happens when the laptop is just sitting on a table. Probably wouldn't be a good measure to stop the drive.

Initially, an object falling will accelerate with g=9.81m/s^2, after it reaches the ground it will de-accelerate to a speed of 0m/s in a couple of ms. You are confusing weightlessness/zero-g with no acceleration.

[Isamu]

I'm talking about the acceleration that the sensor is measuring. If you are holding it still, it measures 1g toward the ground.

If the drive is falling, certainly it is accelerating toward the ground but the sensor measures zero acceleration. This is the relative frame of reference.